Possibly the leader of the party didn't know how bad the coblyns are, or perhaps he wanted to see higher exp/kill, if you get in a party like that again (if you know you will get raped by a mob) speak up, tell them your "exp is fine as it is and exp/hr > exp/kill." I could be wrong but if your getting much more than 500 exp/kill go to slightly weaker mobs, you will kill faster and might only be getting 100 exp/kill less, but exp/hr could be twice as fast.

Oh and if anyone suggests killing coblyns for exp tell them no they have high def and we aren't at release now, coblyns suck.