Join what? You are already in the channel you are facing and /join requires a parameter anyway. Part button is already there in a sense, it's the bottom X at the top right. You have a point with rejoin though there is the short, easy to remember and use /hop command.

mIRC's GUI is probably the best in the business, with Bersirc coming a close second. Why? because they are uncluttered and leave heaps of room for the important bit, the channel window and nickname listbox.

I'm not against a new toolbar, don't get me wrong there though I would like the option of disabling such an addition as the clean look of mIRC is what makes me use it. grin