They tried to make trash mob useful to kill when they added some gil to each kill, but I think it's not enough insentive.
As said before, make it stack myth/soldiery for each kill, and give them all at the end of the dunjon, so killing trash will become useful, and doing the dunjon to the end is a necessity.
But it won't stop "pro speed running" when tank takes 3-4 pack of mobs and the team AoE the hell out out them.