There's been times when on Rush when nobody uses Engineer, and then when one team gets a vehicle it crushes everyone else. Or there's a vehicle and nobody goes Engi. to destroy it. I just wish people would go AT more.
Maybe not underused, but not used well or efficient.
*Edit*
I'm looking at the unlocks, and I don't see very many people using the EOD bot, or the landmines. Makes me want to play BF3 and try to use them. >_<