If talking precisely from the unclog perspective, you must be right. However:
1. As far as I understand, the majority of people use this function in order to save ink from a purging process of their printers. At least, it corresponds to Canon printers.
2. Contemporary printers get clogged less than it used to be. My dye-ink Canon Pro-100 didn't get clogged even after more than year of inactivity (the printer was switched off)! And it happened twice! Some users of pigment-ink Canon printers claim that their printers don't get clogged after a few months of inactivity.
3. Even in a BW mode printers use not only monochrome inks but color inks as well. I'm sure that when printing color images printers will use almost all colors. Surely, some colors may be used in a small amount but it still can be enough to avoid clogging.
Now I have a pigment-ink Canon Pro-1000 printer. It is said that if I don't print anything in 24 hours, it will initiate a self-cleaning process that wastes a few ml of inks. So I have to print something every day. At the same time I don't see any point in messing around with everyday unclog printing when I could print my photos instead. Automatically, ofcourse.