In general, the only people who actually care about spelling on IRC are the people typing. Rarely have I seen anyone complain about someone else's spelling unless it's so bad that you can't understand what's being said. If no one reading what you write cares, then the benefit is very minimal.
I care about spelling and I will point out spelling and gramatical errors to people when I see them on things that aren't temporary. I don't point out errors on IRC because, as argv0 stated, it is temporary and fixing errors has no value there. I also limit where I point out errors in other locations, such as forums. Although posts are more permanent, pointing out errors just derails the threads. Besides, posts are rarely significant enough that errors matter. It's not like a resume or essay where errors can cost you. The point is that IRC really isn't a medium where spelling and grammar matter.
Even though I care about spelling and grammar, I usually ignore the red squiggly lines from spell checkers. In most cases, they are wrong. I also rarely add words to the dictionary unless I get tired of seeing the red squiggly line on the same word over and over. I do, however, pay attention to the green squiggly lines for grammar because those are often right. A spelling error or typo now and then really isn't as bad as horrible grammar, imo. It's very easy to overlook a spelling error when reading something, but bad grammar sticks out like a sore thumb. Even so, I don't think we need a grammar checker on IRC either for the same reason that I don't see any significant benefit to a spell checker. It is temporary and *very* few people care if you type something incorrectly.
Here's an example of how bad spelling usually doesn't make something difficult to read...
Arocdnicg to rsceearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteer are in the rghit pcale. The rset can be a toatl mses and you can sitll raed it wouthit pobelrm. Tihs is buseace the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
Note that no such study was actually made and this is just a common example of how spelling errors don't necessarily make something difficult at all to read. Also note that not everyone can read this as quickly as a correctly spelled sentence and anyone who isn't a native speaker will probably have more difficulty. However, most native speakers can read it as well as a normal sentence.
The point being that spelling errors aren't the end of the world and that correcting them really only matters on something that is permanent, or at least semi-permanent. Other than logs that very few people read, IRC is not permanent at all. Grammar is usually more important, but it's still not important on IRC.
As I said, I actually do believe this is on Khaled's list of things to do. I don't think it's very high on the list, though. Anyone who really cares about spelling should look into 3rd party alternatives because it could be years before you get something added to mIRC.
