I don't see, the spell gets the benefit of the caster's penetration score, a stress die and stamina, the +1 bonuses from loud words or exaggerated gestures, talisman attunements, spell mastery, virtues such as life boost, cyclic magic, special circumstances, and method caster among many others, it can be boosted by arcane and sympathetic connections, it can be boosted by vis, it can be boosted by wizard's communion, the spell gets better with the caster's art scores while the item does not.
The situations are not at all the same.
It makes sense in game that a thrown rock is not stopped by magic resistance, It does not make sense that a rego mentem spell can not be resisted without defining magic resistance significantly differently. What do you expect the player to come up with. I like cool explanations but this is wide open for abuse. I don't think it is worth it.
I understand that you don't like it, but"It's stupid" doesn't really make what is bad about it clear to me. You don't like that magi can create spells the require mastery to be really useful? I think that this is a fairly cool option that I wouldn't want to be made superfluous.
There are other options to Wizard's parry (like here Personnal Wards, Soak, Parry, blah blah blah) it is the sort of spell that is used as a fast cast spont put into black and white so it can be read, I wouldn't expect to many other magi to learn it.
This makes casting a spell quickly, which can be done in several different (IMO) cool ways (mastery, quick items, environmental triggered items, waiting spells, on rare occasions, spells with weird parameters such as until if used creatively) significantly easier and (once again in my opinion)less interesting.