My guess is that mIRC assumes that if you kick yourself, you want to leave so it doesn't rejoin. It's kind of a gray area and different people would want it to go different ways. Some people kick themselves for fun and want to rejoin immediately. Others kick themselves when they want to part the channel and so they don't want to rejoin even though they would want to rejoin if someone else kicked them. Personally, I don't see the need to kick yourself, but that's just me. I'm not sure which would be more popular and as this has probably been like this all along, I don't think it should really be changed without a good reason for doing so.