I want Garrus to be a squadmate in ME3, and I don't think it would make much sense at all for him to leave. I still think all the squadmates will return though, with a few of them doing something important to help the mission without necessarily being a squadmate (like Wrex) and a few new ones. Of course, BioWare can make it happen if they want to... and if I can have Kai Leng as a squadmate, that would almost make up for it. xD But BioWare always writes good stories, so I'm confident that whatever Garrus does in ME3 will be awesome.
To answer Buggirl's question, Garrus probably would have joined Shepard no matter what was going on in his life at the time. If his team was still alive, I'm sure he had someone who would be a decent second-in-command. If he joined C-Sec... well, he doesn't seem to like it that much, so he would have gone off with Shepard again. If he was a Spectre, it would be easy to go help Shepard out. The Council might be annoyed that he's working with Cerberus, but it's possible for Shepard to keep Spectre status, so the same should be true for Garrus if he went to work with Shepard.
Also, I liked the whole story about Garrus' time on Omega, his recruitment, and his loyalty mission. I like the revenge themes too.

A lot of my favorite characters (...actually, I think it's safe to say all of them) are the vengeful type, so obviously I wouldn't mind that it's done often.