Yes, some people love accuracy and correctness - and they learn to type. Millions of foreigners can do it; why wouldn't a native be able to speak and type in his own language perfectly (or at least with one mistake every 1,000 words, for example)? The ones who can't tell the difference between "mandatory" and "manditory" won't care if their typing is right or wrong. Writing a built-in spell checker for a communication environment based on getting the message through quickly while disregarding quality, is simply a waste of human resources. You should use a spell checker for your work, for writing your resume, for academic work, even if you know that you make one mistake in 100,000 words. The fact that people created add-ons for mIRC to turn it into an e-mail checker or multimedia player is simply an example of the power of the scripting language. If one feels like it, he could write a mIRC script to replace the industrial computers controlling the robots in a car factory. This doesn't mean that such a feature is normal for an IRC client. Neither is e-mail checking and multimedia players, there are specialized tools for that.
Extending this application beyond its scope will only turn it into a strange piece of software, like mixing up the genes of a sheep, a shark, a parrot and a cactus just to see what could come up. It is silly to request users to use correct language on IRC, even that some people would very much like to see those abbreviations or nonsense gone from their screens (me included). However, it is stupid to force users to do so - see how much you can determine people not to use excessive colors in channels. And, it is a waste of time to implement something that is not desired or needed by the majority of the users; there are other things on the "to do" list, which have more supporters than the idea of a spell checker.
Remember what happened when Khaled decided to change the "nag screen" on start-up from being displayed once every 30 days to being displayed on each start-up, if the user didn't enter a valid license key. People gave up the improvements and bugs fixed because all they understood was that "I have to pay for the new version, the old one is free." The Options entry moved from the File menu to Tools menu was an annoyance to many people, and so were other minor changes between versions. What would happen if people found out the new version prevents them from sending messages as they did for years, or annoys them with underlined words they mistyped, or even makes chatting feel like writing a document for work? They use IRC to chat free of any restraints, to express themselves creatively, to relax. Throw in a restriction which directly affects the purpose of chatting, and you've got a serious problem.
No offense intended, just speaking my mind.