I had a old school type of trainer who said he did not use mitts because he thaught it would take away a fighters own thinking in a fight.
Yeah a lot of trainers think the mitts get the fighter throwing punches in the wrong areas because of the way the trainer holds his hands, but they really do help a fighter develop combinations, and if the trainer puts the mitts in the right places, its effective.
They can be great tool for accuracy if placed in the exact correct areas.. so a fighter can go over and over moves so they become second nature to him in combat.. I remember mitts being used in the late 70's .. i actually like using my hands at times, because they aren't so bulky... i sometimes just wear 10 oz gloves so i can throw cedent quality shots back to make a fighter move thier head correctly... They are only as good as the methods they are used for...