Yes, but the smallness of the icon would still annoy me as it would mean I'd have to make sure the cursor was over the icon to double click it, and every now and again I'd click another program and open it when I didn't mean to etc....as I said, it's just a personal opinion though.
However, off the top of my head I can't think of a program that's main window is open but at the same time it's icon appears in the tray - the tray is for programs running in the background or that are currently inactive and/or serving another use. Having the mIRC window active but the icon with those would defeat the object.
On top of this, if you have two active mIRC clients and have one minimised to tray, and another active but with the tray icon, it'd be more confusing still (and more annoying IMO) to have to check which one was which.
My 2 cents.