Are you telling me you really think having a spell checker will improve the quality of people's typing?
Yes, if people do not know that the way they spell a word is wrong, how will they ever know to change it. Believe it or not, IRC is one of the best ways to learn how to type properly. Unfortunately, it is one of the best ways to learn to type poorly as well. If people, both new to IRC, and people who have been using it since the Dark Ages, start to increase their typing accuracy, it will be beneficial to them (the user), for obvious reasons.
I hardly think it's necessary to start incorporating something like this for someone who, using puterfixer's example, spells mandatory 'manditory'.
The person who spells mandatory 'manditory' will have learned something from his IRC client, instead of letting his brain rot away on the Internet.
It's not worth it, there are far more important features to be working on in my opinion.
Can you name any that aren't related to scripting. Personally, I don't make scripts in mIRC. I actually use it as a chat client. I would like to see features that benefit me, as the non-scripter user, when new versions are released. (Yes, i do hang out in #mircscripts.org, but no, I seriously don't script)edit:
Furthermore, this would actually be a selling point for me to buy mIRC. It's one thing that no other client I know of, besides one I am in the proccess of building in phpgtk, incorporates.