I haven't been in the "hate Rusty" camp like I've seen some... I'm more in the "yeah, why is he there now?" camp sometimes, though. Originally I got that it was a humanizing thing for Sharon, how her character had been previously... they didn't want to retread the way Brenda Lee was introduced and at odds with everyone for the first season of the show... so they give Sharon the Rusty situation and it helps the people at work see she is a more complex person than they knew from her time at IA.
But... after that was smoothed over, and they aren't playing Sharon vs the squad anymore... they sometimes forget to give Rusty a reason for being around. The other people in the squad are sometimes just in the background, but at least you know they are there working on something for the case... they are well-established (for the most part) by now. But Rusty is still pretty undeveloped beyond the case that originally brought his character to the show.
It'd be nice if they either gave his character some more meaningful things to do OR wrote him off as going to college or something. Nothing against the actor either, he's not being given much to play with really.