I don't regularly use any other chat program, so forgive me if I'm wrong, but do any other chat clients have spell checkers? I don't think they do. "Word centric" programs such as Firefox and MS Word have spell checkers because when you type documents in those programs you're probably meant to be spelling things correctly (e.g. forum posts, wikipedia entries). However, the very nature of "chat" on the Internet means words are often not spelled correctly. In fact they are often severely distorted! So Firefox might have a spell checker, but Chatzilla doesn't. MS Word does, but Windows Live Messenger doesn't (afaik).
If you want to type correctly then feel free (I certainly do), but the point of a spell checker is to prevent embarrassing or silly mistakes in documents intended for an audience that might actually care. Also it helps prevent irreversible mistakes - once you've sent that email you can't correct the typo. Once you've printed off that document and posted it, you can't get it out of the mailbox. Making a typo on IRC doesn't matter, is not embarrassing, and the "live" nature of it means you can simply correct your mistake on the next line. Er go, I don't think mIRC is an appropriate program to include a spell checker with.
Regards,
X-Chat has one.
