I don't know if that's right. I think the best conclusion to draw from your archmage "always" preparing assay resistance is that assay resistance is a spell he generally finds useful to have prepared. But concluding more than that on the basis of the archmage "always" preparing it could be misleading.
For instance, my fighter/mage "always" prepares mage armor, "always" prepares anticipate teleportation, and "always" racks a dimension door and a greater invisibility spell. Does that mean those spells are broken? Does it mean that I'm in a campaign where opponents constantly teleport in next to us and start attacking? Does it mean that I'm always being grappled? No on all counts.
My experience playing wizards is that both I and other people generally make our characters good at a few strategies that are generally applicable and usually have the same core of spells prepared. Even when we get advance information and a chance to prepare, we generally only switch out spells around the edges like a magic player with his sideboard. Thus, if we prepare a spell, odds are pretty good that we "always" prepare it whether it's magic missile, mage armor, false life, see invisibility, haste, fireball, dimension door, greater blink, banishment, bigby's grasping hand, anticipate teleportation, or assay resistance.
Similarly, my experience is that a situation doesn't need to happen all the time for a high level wizard to spend a few lower level slots preparing a contingency plan for it. My fighter/wizard rarely uses his bow but he often casts an extended greater magic weapon on his arrows just in case. Because if he does need to use it, he'll want the greater magic weapon. Similarly, I've never actually had enemies teleport next to him and attack. But I keep anticipate teleportation up just in case. Some day it will happen... and he'll be prepared. I even prepare spells that I haven't used in three or four levels (like greater invisibility) because they're good spells that could be the right tool sometime and if they're not... that's what Arcane Strike is for. It certainly doesn't mean that I don't think other 4th level spells are worth my time.
As for assay resistance itself, I think it's a very good spell but I'm not convinced that it's broken outside of an environment that severely limits both encounters per day and creatures per encounter.
Saeviomagy said:
OTOH, that archmage could simply memorise spells which don't allow SR...
Those top-tier spells tend to have something holding them in check IN ADDITION to SR. Like a range of touch, or expensive components etc.
Otherwise EVERY MONSTER AND BBEG would have to have SR. They don't.
So - if your archmage ALWAYS prepares assay resistance, then it's because he obviously doesn't feel that any of his other 4th level spells are worth the time. Which would seem to suggest that every opponent and his buddy has SR. Which in itself is a "screw the magic user over" sort of scenario. Having a spell to help with that isn't a terrible thing.