Why not? You get exp for damaging monsters, so if you do more damage, then you get faster exp. Now I didn't say MORE exp, just faster.
Heavy Cannon killing 10 Launchers in 40 seconds gets 100k exp.
Melee killing 10 Launchers in 10 minutes gets 100k exp.
So technically AOE doesn't get you more exp, it gets you the same faster.
Fixing APU frequency/damage and melee everything will just enhance the experience for players wishing to use those classes/weapon types to play the game.