I've had this happen several times recently, although maybe not in precisely the way you mean, Crothian. I tend to look at it with interest, wondering what cool reason the GM has for bringing in this character. But in my experiences it hasn't been the GM's former favorite PC that he decided to introduce. One of my GM's ran another campaign in another system for several years, and when he started his current campaign he announced his intention to introduce elements from his old game into the new one. Since the new one isn't set in the same locale as the old game, he hasn't had the actual PCs from the old game show up. Instead he's had what one might call "alternate reality" versions of the former PCs.
Another GM is doing something similar, but his newer game is taking place in the same setting as his previous campaign, so he's having our old PCs from the other campaign turn up occasionally as NPCs. My old character hasn't shown up yet, but we've already seen the former PC of one of the other players several times. I find it very entertaining, like an in-joke for our group, or like having characters on tv shows do crossovers with other related shows. Since both GMs are using other players' former PCs, it's interesting to see what they do with them, and I don't worry that they'll try to turn those characters into GMPCs.