This is actually more of a question to Rounin... I was under the impression that using UTF-8 would change how ascii characters looked simply because you would have to change to a font that supported UTF-8 (say Arial Unicode MS?) and that would change the way characters looked. There's also the fact that users would have to have a full UTF-8 compatible font installed (Arial Unicode MS is 23MB) and using such a font consumes more system resources.
Please correct me if any of that is wrong.
Let me say that I'm all for mIRC supporting UTF-8, It's a royal pain to switch between languages in several of the different channels that I idle in. It also makes a certain channel where multiple languages are spoken virtually impossible to follow. However, if I am correct in the above assumptions, I think it would be a very bad to have mIRC default to UTF-8 (or at least default to require using a full UTF-8 compatible font).
I've tried both X-Chat and Chatzilla which support UTF-8, but neither one has the features of mIRC. So what's the end result, X-Chat for the multilanguage channel, mIRC for everything else.