Most mice with extra buttons and stuff come with software that allows you select a function for each button; one of the choices is "default", meaning that it will do whatever it would do by default in Windows (this really only applies to the usual middle button/wheel; there isn't any 'default' function for extra buttons). The default operation of the middle button is certainly (meant to be) programmable. If "default" isn't selected in the mouse software though, mirc (or firefox etc) has no control over the button, so there really is no problem. Middle click customization is in fact a very popular feature in many programs.