What kind of XP system are you implying that they use now? I guess i never could never imagine anybody would use any other system than splitting xp among your party members? If you are right then i agree with you that it needs to change ASAP, whether they implement 6 party members or not.
The full experience from an enemy (or exploration) is given to all party members, not divided between party members. At least, this is how earlier patches and DOS1&2 worked...it's possible that this was changed in a patch but I haven't heard anyone mention it being changed.