Is Imperial Droid a useless character?

Saddly, yes.

Probe with Gar Saxon lead and ISC is one of my favorite 3v3 teams. I find that they can take out GL teams (Assuming they are all at 1% health). Although I am currently reconsidering in favor of HK lead with IG86 and IPD.

Extremely niche. EP instabomb for example

Grievous nuke team, if you have the other members

Are you kidding? Ipd is a good character. I'm not talking about the self-destruct capability, but the mass buff dispel with target lock, which can't be evaded. You can consider that skill a cheap version of Mara Jade's mass dispel special. It works wonder in a Palp-Vader team vs. Padme for example.