I guess your definition of invasive is a little more relaxed than mine. I find underlining words in red to be heavily invasive, especially given that a good 95% of conversations I've seen on IRC (purposefully) involve words/spellings that would never be found in a dictionary. Off the top of my head I can think of at least 1 or 2 occasions *per line* that would flag a spell checker (think 'id', 'youre', 'hes', all the obvious 'lol' stuff, and then all the geek conversations I have that involve file paths, acronyms, math formulas and plenty of code snippets <- this is typical of IRC conversations, imho). That's a lot of red underlines to be looking at. I'd rather not spend my time adding words to a dictionary when I could easily just be ignoring intentional mistakes.
Now of course some people might actually find this useful. I have a good feeling that the percentage is quite low. All I can hope is that it would not be a headache to implement should K decide to do so, should definitely *not* make "suggestions", and should definitely disable-able (because that's the first thing many would want to do).