I personally don't see what the big deal is. I'm a GAIM user too and I think its spell-checker feature is handy - it's not intrusive and it's not annoying. If you still don't like it, simply disable the ability to use it in the preferences! Sheesh.
Anyways, now to address some of the arguements...
-> Different languages: Maybe for english speakers it's difficult to grasp, but there are really many different languages. In Belgium there are 3 official languages, and if you have something remotely to do with Internet you also use English. So which language would you set in mIRC?
Ok, multi-lingual users could be a problem. I think perhaps having the ability to specify the language of the particular channel/private chat would be handy. Perhaps some kind of drop-down menu somewhere. Or, in the mIRC preferences, a list box which allows you to select multiple languages, based on the dictionaries that are already installed or, if none are installed, mIRC could download the dictionaries which are needed from the aspell/ispell/gtkspell websites.
-> Dialects: On IRC, dialects, regional or spoken language are used a lot. In Dutch there is a difference between written language and IRC language. Yes it's not correct, but it is widely accepted. There are still errors (typo's and incorrect grammar for some things), but some errors are intentional for a more fluent or natural message.
I have never seen a spell checker that works perfectly with this language, I might even look at it for 2 seconds...
Fo this one, I think the only viable solution is to either add the custom words to the dictionary or just plain ignore them.
-> "This may not be true, when they see a mispelled word, chances are they'll try and correct it"
No, chances are they'll get annoyed by that wavy red line stuff and get a client that doesn't act like a school teacher.
Then disable the feature.
-> Spell checkers suck: they often give wrong output, both overlooking errors and marking errors where there are none. Same for proper names, nick names and stuff like that. I'm not going to add hundred different nicknames to have a rather clean looking mIRC.
I personally have NEVER had a spell checker suggest the wrong word to me or an incorrect spelling. Grammar checkers, on the other hand, are a different story. As for nicknames, perhaps mIRC can be set so that any words in the nicklist are not checked.
Sure, the feature can be scripted but then so can most features...