I never found much of a use for Mordin. While hos powers are useful he had the distressing habit of dying rather often. Even on normal difficulty I'd often fine myself running out of medigel for the amount of times I had to revive him. Jack too for some odd reason I mean you'd think she'd be pretty tough but that girl really has a glass jaw.
Speaking of Jack (and I may have brought this up before) but I was rather disappointed with her. I mean from the trail of destruction she leaves behind in her recruitment mission they make her out to be a neigh unstoppable biotic battering ram. Though I appreciate that any squadmate that really had that power level would totally break the game, perhaps they could have used some kind of sustained ability that for a short time (and with a long cool down) puts her normal abilities into overdrive. Indeed a lot of the characters could have done with something similar. Grunt's fortification and Legion's geth shield were a good start but I think Garrus could have used a "focus" ability that briefly increased his accuracy and rate of fire. Not so sure about the others but something to make squad selection a little more involved.