I'll try but to be honest I'm not sure what I have not explained clearly (asides the fact that my English may be far from perfect ^^).
In our saga we decided that you cannot use a pink dot defense. Pink dot being "any easy magic you apply with the only purpose of the [whatever is dangerously coming for your life] to fail against your parma/magic resistance due to the fact that this easy magic will have no penetration".
We also decided that if an effect is just a bonus (for example sword on fire) then the sword could still hit, but if the effect is a big part of the situation (for example: the sword is magically created) it would be resisted (if RM > pen, of course but that's the basic premise of this discussion).
Callen's gave the example of invisibily in his post, and an invented magus "Now you see me" who would have an invisibility spell he would have masteries: MR, FC and MC.
He says that to counter that, the "invisibility" is considered a bonus, thus the magus Now you see me is not very lucky with his "overal perfect" defensive technique.
In our saga, we specifically consider invisibility (or anything who would allow for an attack to happen without a defense roll or any chance to fastcast against hit if you fail the awareness/whatever roll [which for invisibility is quite impossible to spot unless you have second sigt]) to be something which is resisted.
It is specifically the case of a NPC which is permanently invisible but render himself visible the (less than a round) time it requires him to hit. In roleplaying description: the blade becomes visible for a brief time and then hit, then invisible again.
If the NPC (or any one obviously) could remain invisible, mundane assassins would be the most fearsome danger for magi, because they could hide anywhere when the magi could not allow to be seeing the invisibility at any time (for different reasons: fear of warping, inability to do it, too high invisibility level to match with an InIm spell, etc.).
Since we decide that you cannot defend (even with fast cast or lightning reflex) from something you have absolutely zero chances to spot, it would mean for the average NPC an average attack total of: 12 (skill+charac+weapon) + 5 (fatigue spent... you want to kill the magus in one hit) + roll attack (average 5.5 let's not delve into botch and 1' rerolls) + dmg (let's say 7) which mean: 30 dmg vs a soak.
Since magi are only in armour while they expect danger, do not usually ward themselves against metal (because armour is bothering, because they use magic item made of metal, because they are walking on metal...), we can assume a full soak of 10 + dice. Which means that our average warrior (and if that was the setting, the warrior who would be sent to that magus is not an average guy, but someone with +3 or more weapon, with at least 7 or 8 in the skill etc. thus having largely more than 30 dmg in his first attack) will probably heavy wound the magus or incapacited him in the first round.
If heavy wounded, the magus cannot really ripost: he still cannot see the foe and even if he has defensive spells, he is wounded badly. If he has no defensive spells, he cannot escape spontaneously because he cannot fatigue anymore.
If he is incapacited, he is dead in the next round.
Forcing the blade to become visible to strike (which may, in the most cases, mean that the attacker needs to recast/re-use his invisibility spell thus losing the 2d round) gives a chance for an awareness check and fast cast response, or if you have lightning reflex, the use of the virtue itself (you sense, and must react before the SG says what happens).
Additionaly, I was saying that "Now you see me magus" would not be allowed by the various SG's (due to the alpha SG veto-ing this concept) for NPC or PC. The reasons would be among those:
- pink dot is not a good defense in principle (callen's explanation)
- purpose of invisibility is not to protect oneself against incoming attack
- you cannot use a spell with fastcast (often required to prevent an attack by arrow you didn't know was coming for example) and multicast in the same round (I think that this is just RAW in fact)
Since no player would create a magus concept the SG would just ignore, nobody would do the magus, thus the "paradox" (invisibility is not a pink dot thing - vs - now you see me magus use it as a pink dot defense) never exist.
Is this perfect/logically perfect/"magically perfect"? no.
Is this playable and undertandable? yes.
(And I think Ars being a game, the latter is preferable to the former.)
Can players abuse it? certainly.
Does the SG allow abuses? never. Try it and you will lose your character without warning. The game is a contract of fair entertainment between us. If I want to abuse, I play a solo PC game... until the point it is boring .
I hope it made my position clearer?