I doubt skins are going to signifigantly increase mIRC's user base; because it would imply the only reason they don't use mIRC is do to the lack of skins. To my knowledge, there are no other popular IRC clients that support skins. So where would these people come from? They'd just say "now that mIRC has skins, I think I'll start chatting on IRC?" I really doubt that will happen.
