Well...precisely...it won't stop them typing like that.
I think it's a nice idea, but not practical in the IRC format. Many people don't touch type, and it causes a whole lot more hassle when they have to keep looking up at the screen to see if they spelt something wrong, and then to replace it. Even if you do touch type, you're not going to want to have to keep correcting things. Unlike Microsoft Word, you have to keep up with the rest of channel screen scroll, especially in a channels that have 60 people or more.