I concur. Having UTF-8 (input and output) enabled by default would at last fix the codepage problems that have plagued IRC since its inception.
Simply having it as an option (but turned off by default) will not help much because most casual users of IRC simply have no idea what codepages are or how to fix them. Trying to teach them is for the most part a futile effort, and frankly unnecessary. If mIRC used UTF-8 out of the box, everything Would Just Work (tm).
I disagree about putting another option in the installer though; UTF-8 adoption is IMHO far enough that its encoding can be made the default, no questions asked.
