Its mainly just to unlock some zerg portraits at the minute, I am not very good at the multiplayer... But I haven't had any experience yet for the games I have won. Is it only at the end of a league that you get the total xp?
Also, do I get xp for playing games against the AI using the matchmaking system where you progressively fight harder AI?
Im not sure if they will give it to Wings of Liberty at all to be honest, it is an feature that might be meant to attract the crowd who seeks 'progress' when they play beyond improving theyr gaming capabilities and archievements.