I think Vader is there as a toolbox choice, to push that one more point of damage through on a vital moment - man, I could have ended a recent game against Corran if I'd've had him in there instead of Isard. I'm not even sure how much damage Isard really saved me from anyway.
Palpatine is... he's a crutch. He really is. Yes, he's good, but he's not the only good thing we have, and he's the good thing that everyone EXPECTS us to bring.