We already know that spell checkers are not perfect. You don't need to explain the downsides of using a spell checker. It doesn't change the fact that it is a popularly requested feature for mIRC and one that many people including myself would find useful in spite of those downsides. Spell checkers may not be compatible with the way you personally use IRC, but that doesn't mean it wouldn't be useful for others.
On a side note, as your own image shows, Chrome doesn't underline any part of "u r gr8", which sort of ruins the point you were trying to make there. In any case, if you are the type of person who uses terms like "gr8" or "ur", then you probably aren't the type of person who would enable the spell checker anyway.
