Did I say it was an important upside? No, I did not. I merely said it was an upside of not issuing an errata, and it is.
Adding errata always means extra work for players, which gives it a downside. Very frequently the benefits of an errata outweigh this downside, but that doesn't mean there isn't a downside.
Actually, it is. I copied and pasted from what he wrote:
As part of this process, the magus may add details that enhance the Shape and Material bonus of the item. These details give an additional bonus to all the item's existing Shape and Material bonuses equal to the creating magus's Philosophiae score, for the purposes of enchantment.
Now, in your quote he's talking about editing it. But he's not giving any specific version of the edit. Quite clearly, though, he's still talking about "for 'enchanting,'" right? So he's saying he's not looking at changing the "for the purposes of enchantment" part.
It seems to me that clarification is needed. You have a choice between what you had originally intended- that Verdituous runes enhance an existing S&M bonus, or something approximating how it is most commonly played- that Verdituous runes act as a S&M bonus. The second is clearly the more powerful option since it has the option to be used with existing S&M modifiers, and the ceiling of Magic theory limits the potential significantly if they are required to stack. The popularity suggests that the power is desired whether balanced or not, but it is ultimately your call, that's one of th downsides of your position, having to make the call (balanced in part by getting to be the one who makes that call. Blessing and a curse and all that). I do think clarification would be important, as I have had people quit games when they discovered that the virtue they were initiating was not the virtue they thought it was.
But presenting them as a fait accompli does not leave room for discussion. Trade-offs everywhere!
There does now seem to be a consensus that a clarification in this case would be good. This makes me happy, as, although I know that the sunk-costs fallacy is a fallacy, it is still annoying to write them off. For clarity, here are the two options for Verditius Runes that I am currently considering, the first of which is the version above, for reference.
As part of this process, the magus may add details that enhance the Shape and Material bonus of the item. These details give an additional bonus to all the item's existing Shape and Material bonuses equal to the creating magus's Philosophiae score, for the purposes of enchantment. These bonuses apply in the season that the details are added, as well as in the future. Other uses of Shape and Material bonuses, such as the casting bonuses from a talisman, use the standard bonus. Other magi refer to these details as Verditius Runes, but they are far more complex than that suggests, and do not normally look like actual runes. The total bonus from Shape and Material and Verditius Runes is still limited by the magus's Magic Theory score. Other magi get this bonus if they instill appropriate powers into an item created by a Verditius.
Here is the alternative:
As part of this process, the magus may add details that make the item more suitable for enchantment. These details give the item a Shape and Material bonus for "enchanting this item" equal to the creating magus's Philosophiae score. This bonus applies in the season that the details are added, as well as in the future. Other magi refer to these details as Verditius Runes, but they are far more complex than that suggests, and do not normally look like actual runes. The total bonus from Shape and Material is still limited by the magus's Magic Theory score. Other magi get this bonus if they instil powers into an item created by a Verditius.
Read one way, 2 S&M giving the same bonus would accumulate twice the Philosophiae score. I think all/each should not be used here.
"additional bonus beyond the item's existing Shape and Material bonuses"
I prefer the first one. I like the idea that without casting spells or spending in a lab, a mage, or even a non-mage with decent MT can try to guess the effect of a magical item. The second option will remove that.
I prefer the first option.
The second one feels a bit too "artificial". Like something which is written just to fill in a hole (in the rules) with a particular shape without having to change anything else. Which I guess is more or less exactly what it is.
There is one little unclarity though.
The total bonus from Shape and Material and Verditius Runes is still limited by the magus's Magic Theory score. Other magi get this bonus if they instill appropriate powers into an item created by a Verditius.
Which magus' Magic Theory score limits the bonus? As it is written "the magus" can only refer back to the Verditius magus who added the Verditius runes, since no other magus has been mentioned thus far.
But logically, if another mage instill powers into the item, the bonus ought to be limited by their MT score, not the Verditius magus' score, right?
Just switching place of the last two sentences might fix that, maybe.
The first option effectively maximizes the Shape & Material bonus to the Verditius' Magic Theory. The second option ensures that Vertiditus always have a Shape & Material bonus that is at a minimum equal to their Philosophiae.
But it's not difficult for a Verditius to get a Shape & Material bonus -- that's another part of the House Virtue. If they want an amethyst bonus for a wooden item, or a silver bonus for a tattoo, not a problem. So a Verditius already has a good way of getting a mimimum bonus from Shape & Material; it would be less redundant to have this instead maximize their potential bonus. Which would be option #1.
One of my concerns with option 1 is how useless Philosophae will be for the Verditius outer mystery. It's relatively trivial to get +4 S&M with a single component for a normal magi, which means a magi outside of the gauntlet won't need Philosophae at all most likely. And by combining components, it keeps staying relatively easy too except for invested items, until you get a very high MT score. Except that a Verditius needs a craft score, which normal magi don't need. Hence you end up raising two-three Craft skills to get versatility on S&M, and you ignore Philosophae until you get to elder runes or Items of Quality.
It provides flexibility, not more power, there. That has always been the case. With MT 3+, as most magi have, it's almost always trivial to get an S&M bonus for a lesser enchanted device or a charged item. But the Verditius can do it without needing so many specific materials, just one appropriate thing. Philosophiae is actually slightly more useful there than it had been due to fixing the future tense later on.
With invested items Philosophiae shows up two-fold. One is what you mentioned about later having high MT. Another is flexibility. A Verditius could choose components that add to different things rather than stacking them. So the Verdtitius item can get a big bonus to many different things, so long as they have decent Philosophiae. I've usually found the point of an invested device is multiple effects anyway, such as wanting a mental trigger. So this will still put noticeable value on Philosophiae.
So as far as I can tell, this doesn't make Philosophiae's utility noticeably different than it had been; perhaps slightly stronger actually with the lack of future tense. Perhaps this is why the canonical examples at gauntlet have only Philosophiae 1 (core book and Gwydion in MoH)?
As for raising multiple Crafts, you commonly don't need to do that since you get to use the highest relevant Craft. For example, a swordsmith might decorate a sword's hilt with all sorts of gems, stones, wood, etc. without building up a second Craft at all.
Since I'm now playing a Verditius character for the first time, it's been a bit of an eye-opener as to how the rules really silo them off to one specific thing, to their detriment. Merinita, for example, are probably played by people interested in Faerie things, and Merinita get advantages to Faerie stuff that does not require additional investment to realize. Verditius get saddled with casting tools (probably for legacy reasons) and need to sink experience into Craft(s) and/or Phillsophiae for some minor bonuses to enchantments.
Honestly, the option to sink lots of experience into Philosophiae -- in addition to Craft(s) -- so I don't have to worry about Shape & Material doesn't result in an enchanter that's at good at enchanting as a Merinita is at faerie stuff. Anyone can get a similar Lab Total by being clever picking raw materials. Being able to go above the Shape & Material bonus is something that would set Verditius apart from other enchanters who aren't saddled with the experience-sinks.
And, as a Storyguide, giving the Verditius an incentive to ignore finding raw materials for a good Shape & Material bonus is just another reason for them to nope! out of Stories and stay secluded in their laboratory/workshop. Which is not really good for a fun game.