However, they would need to implement a system of rejoining the BG, otherwise it is pointless. You can already kick an AFK'er from the BG if you're party leader, or ask them to.
If not altering the XP gain, at least making a system where you can report a player for AFK, and subsequently get them kicked and replaced would be a good idea. Apart from the various AFK's I've encountered (90% of bg's I've actually played) I've loved PvP so far.