I enjoy the loot drops from killing enemies, and they'd have to make quest reward less exp in order to balance progression. I assume they made enemies not give too much exp in order to prevent people being under leveled if they happened to be behind someone. So, that person would get all the exp from kills, and you would get less exp for completing the mission, putting you behind.
Pointless? I dunno i have fun fighting dudes.... I've done devils lair like a dozen times by now and its STILL fun. Cus i like fighting dudes in this game. Granted i wish the two bosses weren't so bullet spongey