You should be rewarded SP/XP for the quest mob kills. I don't want a lump SP/XP reward at the end, But if the quest has me kill green even match monsters, I expect to get the SP/XP as if I were killing a normal grind mob. If exploitation is the concern, only allow flagged questers to be able to engage the monster, once they met the requirements of the quest goals they can not attack said monster any longer.