The problem with using Word's dictionary is that alot of people don't change it to suit their own country's version of English. At any rate Micro$oft still get it wrong. I personally like the idea of a spellchecker that can actually spell, though at the other end of the spectrum it is pointless having one if either the dictionary is wrong or user settings are not accurate.
All up, mIRC really doesn't need one. It is easy to script a small one for commonly mispelled words like "teh" (the), etc. It'd be a good way of putting a 'talker' script to some good use.