Jay losing his powers doesn't mean that he can't be a useful part of the team. He mentors Barry and teaches him how to develop his (rather awesome) new lightning bolt powers, but when it comes to fighting the bad guys he's reduced to being a punching bag. While he can still help, he's obviously going to be a lot more effective when he does actually have his speed back, especially when it comes to fighting Zoom. Although it'll be interesting to see how he copes without them for another couple of episodes, you'd think they wouldn't want to have such a big character - another Flash! - around for too long without powers. Given he already had the ability to turn his speed into lightning bolts, it also poses the question of what other abilities he has that Barry doesn't?