It actually also depends on the computer in question. Opening the plugin takes some time, and for me it’s physically faster to move my cursor to a plugin window and press “process” than pressing a key to open, wait for it to load, pressing enter to process, wait for it to close.
Further more, as I explained either here or elsewhere, I am suspecting that there is other behavior specifically related to iZotope’s RX software that would benefit from the ability to leave a window open.