Shingen's Burst Streak can have some usage.
Herc's Overlimit Buff is also not to underestimate.
Andromeda's Defiance Buff can be of use with a taunt/cover target.
Solomon's CD Reset is incredible useful.
Joan's stackable Def buff shine's in longer battles.

Notable mention:
Arthur's MEX are both not so bad. Her Crit buff is raid wide.
Sieg's Dmg upper limit Buff can have some usage as well.

In the end, most MEX skills can be of use, just like Souls. Unless you narrow your view on only a specific Team-Composition, then of course you will only use 1, maybe 2.