I've done the EX fights enough times to tell when people aren't pulling their weight. I play the majority of classes so I know what to expect from them.

Picking out who is just there for a carry is the hard part for DPS though.
Like Titan EX with a full I85 group and Titan gets three MB off before jumping. I don't like to see more than 1 MB with a group that geared. But at 3 there is an obvious issue.

I've seen more parties fall apart due to not knowing where the exact issue is. Aside from Obvious Tank and Healer issues. Being able to see others output and allowing a chance to improve before replacing has saved more parties I've been in than ones it has ruined. (Actually, immediate intervention has worked much better than waiting until frustration sets in)

I think a DPS tool would be helpful if it worked like the rest of the voting tools. Everyone should vote if they want it to be displayed and majority dictates.