I think part of the reason why Carter has a new romance every season is that he's a leading cast member and has to be featured prominently. But since he has a new girlfriend in almost every season, it's almost like the writers have handed the character a big neon sign that says, "I'm No Good At Relationships! But It Shouldn't Bother You!"
In less than two seasons, Carter has broken up with Abby, had a child with Kem, broken up with Kem, and now he's eyeing the new social worker. And of course he's been to Africa, but it seemed like that was a good way to get him to break up with Abby and take up with Kem. I don't think the experience changed him professionally, whereas the experience was obvious on Luka (he doesn't like unnecessary tests etc.)
I'm surprised more wasn't made of his promotion last season from Chief Resident to Attending. We only found out in episode 10.17 The Student when he was with Kerry and the other Attendings discussing the med students for the Match.