Antimagic can be offensive as well as defensive. Forcing Miscasts on enemy wizards, outright ripping spells from their minds, taking power they use for spells and using it for our dispels, etc.
Sure it can - but there is a question of what is actually practical.
We are speaking about talisman, and while Dwarves are incredible at that things, external tool have inherent limitations. Such as lack of precision. Lack of range. Antimagic being more of a field rather than anything possible to shape into projectile. For example, I fully expect antimagic to shot down
all spells Mathilde have on at the moment. Shadowsteed? Puff. Aethyric Armour? Puff. And what's more - she cannot cast magic when effect is in play.
Antimagic is good in situations where disadvantage of stripping protections and potentially sending mathilde tumbling from suddenly non-existent horse is outweigthed by benefits. That happens mostly when Mathilde messes up and miscasts - because then we
know that effect will hit and will be painful.
Other situations... let's consider few types of situations:
a) Enemy caster about 20 meters casting a quick, low-level spell.
b) Enemy caster about 100+ meters of casting advanced ritual-like spell.
c) Various enemies in an army casting spells.
d) Dhar-tained place with unknown effect.
That should cover majority of encounters. Sure, not all of them, but most common ones. Now let's consider how antimagic would fit into that:
a) Situation is best solved by shoting few bullets into brain of a enemy caster. That is a reason why Mathilde bought revolver. Here, using antimagic is not worth stripping Aethyric Armour and/or Shadowsteed.
b) Very unlikely that field can reach that far - and either way casting Pit of Shades solves majority of problems. Mathilde also have her own dispel either way... which would not work if she tried to use antimagic.
c) Once more, very unlikely that field would reach that far.
d) Here it is useful... but it also leaves Mathilde powerless. Best approach may be analyze place and then destroy it (or call priest to consecrate it).
So yeah, antimagic is incredible boon - but in majority of situation there are equally viable alternatives to antimatic. The point where it shines are miscasts. Mathilde will be casting battle magic sooner or later. Miscast in it can outright kill her and end the quest. That is what is important to avoid - once we have it (and we will with 15 points) we can focus on general protection.
Hence "Protection/Antimagic".