It's common sense: Those users who are currently using other IRC clients that like skinnable chat clients might switch to mIRC if it was skinable.
I personally think a skinable mIRC is just nonsense, but it could have some sort of positive outcome for mIRC and could benefit Khaled.
Implimenting it would, at least I think, seem to cheapen it. But if the option wasn't made too apparant, and mIRC didn't "have to" have skins loaded, so extra resources would not be taken up.. Why not?
The question is, how much more of a user base would mIRC gain, and is it worth taking the time to add this option for that increase? This question is for Khaled to answer for himself. It is not ours to judge which way he decides as well, because I'm sure he'd have a good reason for whichever route he takes. He has, after all, had the good sense to create a very very useful program that has become the most popular IRC client..