Okay. Since my original ruling of "Let's leave it raw for now, and if it becomes a problem we'll deal with it then" didn't go over so well, and since this seems to be a highly contentious issue, I'm going to leave it to a troupe decision.
I'm posting a poll, that's going to run until we have a clear majority for one option or the other.
The poll allows re-voting, so if you change your mind you can change your vote.
(edit - for some reason, the post with my original ruling, although it shows up if I do a search the thread thingie (search for the word "fairness"), doesn't show up on the page. Should be on page 5 of the Pre-play Discussion thread. Which moves my annoyance from people still fussing over it after I made a ruling to the forum for making posts disappear)
The app I use for forums does not display a poll. That said, this proposal introduces a huge balance issue, essentially necessitating the presence of a PeVi specialist to overcome lesser and middling creatures, and making tough creatures invulnerable.
If you're going to leave major house rules up to popular vote, when people's concepts of balance are clearly skewed by minmaxing munchkinism that no one in this saga is doing, please at least be willing to re-visit the vote later on, after we all get steamrolled by a 10 might demon.
To be honest, I'm in favour of going with the rules as written myself (i.e. no Might Soak), but I'm getting very strong and rigid opinions on both sides of the matter. I thought had posted a ruling on Friday, which isn't showing up anyway, saying that since I couldn't get a consensus, that we'd go with the rules as written until/unless something comes up that shows how wrong I am. I think that's what pissed me off last night, more than the spending more than an hour and a half going through the scenarios using what I thought were reasonable examples, was thinking that I was being blown off on the matter. It wasn't until this morning that I realized that, hey, maybe my post got scuttled somewhere.
Right now, I'm leaning very strongly toward leaving things with the status quo, with revisiting it later if circumstances warrant. On the other hand, I don't want to be a dick about it and am open to someone convincing me that the rules are so borked that going with a Might Soak is the only reasonable way to proceed. And, right now, I'm not seeing that.
I have no truck with that. Just as long as Duncan doesn't show up with his diabolic master and wipe the floor with us without adequate warnings. Players sometimes miss foreshadowing, though. So, don't feel free to drop a clue-by-four now and again.
Ars Magica 5th edition shifted the concept of penetration from total casting total to casting total - spell level. This shifts the game to concept of weak effects penetrating or high power effects really needing boost, arcane connections, multipliers, etc....
This means low level spells penetrate easier. It is easier to get small effects against magic resistant foe. Most low level effects are minor and limited, might stripping is the one exception. Level 5 is as good as level 15 if you can get the 5 past MR.
The concept of the might soak shifts this bias from high level spells not penetrating and to low level spells being ineffective so that you need medium rank spells with penetratation boosts. (to low they do nothing, to high they don't penetrate).
I don't see that we need to do this change to raw. The limit on multicast house rule cures a lot.
The reason Soak works for physical combat is that the higher the attack, the greater the damage inflicted. It's a flat total attack less the total defense, and the greater that total is the more damage it does. Thus Soak works-- if you can't hit the defender well enough, you won't do damage, but if you attack much better than their defense, it won't matter how much armour they're wearing, because you'll still get damage through.
The Soak mechanic doesn't work with Might and PeVi because of the 5th ed changes to casting totals and penetration. If you cast a lv 5 DEO with only 10 ct, it deals the exact same damage as when cast by the guy with a 50 ct. This is NOT ANALOGOUS to physical combat! If the greater casting total translated to greater might stripping, then PeVi Soak would make sense. Fixer's proposal might work okay in 4th edition, but really, really does not for 5th.
There are Qualities in RoP:M that increase MR against might strippers. I think this is what Fixer is looking for.
When you want a beastie to have staying power, you use mechanics like natural invisibility (caster must perceive the target to affect it), secrecy and subterfuge (the demon is masquerading as a hermetic apprentice and gives you no reason to suspect otherwise until it's too late), manipulation (takes hostages, controls your allies into attacking the magi while he escapes), et cetera. Demons should be fairly easy to dispatch in a straight up combat situation. That's what counterbalances the fact that it takes a whole hell of a lot of legwork to get them into a direct combat situation.
RAW for me, I prefer RAW or official errata, or if there is an obvious mistake (see the two new spell effect levels in the House Rules section). The arguments are solid on both sides, but perhaps a lot of effort is being put into an issue that might not come up.
At the end of the day, in Ars it seems that there's always a way to one shot some kind of enemy. And to be honest Ars isn't different from most RPGs in that regard.