Totally makes dramatic sense, particularly with all this talk about "personal sacrifices" and Ianto's family playing a part in the story. I think Ianto does it, in part, for Jack, who wants him to get away from Torchwood and have a normal life. It will be selfish and brave and terribly sad all at the same time. And it will free Jack up to hook up with Gwen or go haring off with the Doctor. But because the powers that be at the Beeb appear to like Gareth, it probably won't be permanent. Or he'll get work on another show, whichever.