I think it was answered at the very first UFC in the fight between Royce Gracie and Art Jimmerson (see below).
I can't believe Jimmerson came in with a boxing glove on
Ok he might not have been the best boxer, but he was apprently the no.10 ranked light heavyweight (IBF) at the time but I struggle to think of any boxer who has achieved any real success in MMA and those that have had any, have add to develop an all round fighting style.
There's always the puncher's change but really it's a no contest before it even starts!