This has been suggested before, quite a few times. One such is here
and another here
I don't have anything hugely against it, but I think it would get annoying...people requesting webcam chats when you don't have a webcam, and sadly (and this isn't specific to mIRC/IRC), having opped in kid channels before, I know how this can be abused in a perverse way.
I also do think it detracts from the whole idea of "text chatting", and is a novelty feature more than anything, perhaps even tacky.