Well, Joan's HP relic as well.
Assault is nice and all, but if one uses Joan they're pretty much committing to turtling a nuke, and besides the skill buff on her assault is ass.
If only atk break -30%...
Following the Illuyanka debate...
If I argue that having the option to pick your eidolons for some pretty good summon effects over having the 5% bonus is better?
Skills that come to mind are Vine Hraes (yes, for them water mains without the full def break. Get the axe so you don't need those 2 homies) Ouroboros Icarus Sphinx... etc. Not to mention the stat balancing with and without Illu.
In other words, it's way less suffocating to optimise a team without Illu in it. Besides, Ikki makes a pretty good point on friends, running an Illu main kinda suffocates your other friends in their eidolon selection as well.