Hundreds of people wanting something is a small drop in the bucket considering the many thousands (hundreds of thousands probably) who use mIRC. There is far more support for emoticons (or images in general) than for spell checking and that's still not part of mIRC. Hundreds of people isn't much considering the number who use mIRC, so that's not really a great reason to add a feature.
If you read through the thread, I think you'll find that using external dictionaries is already what people are suggesting. No one is suggesting that mIRC create its own dictionaries. Or if they are, they're crazy. That would require way too much time for Khaled to maintain many dictionaries for many languages.
As I pointed out, Khaled most likely (this is a guess) has this on his To Do list. But don't expect it to happen anytime soon. For one, mIRC updates that include major features usually only come out about once a year. And spell checking is not a critical feature that would benefit the most people, so I am sure it's far down the list (just like emoticon support). I'm sure it will happen some day... but it probably won't be for years.
