For some of these it may be a quality issue. With less material to deposit in the cartridge it may not apply evenly. Lots of times this could be fine but it might not always be the case.
That's possible and understandable for high volume printers. But for personal/desk printers they should have an option like. Cartridges are running low on toner, would you like to enable degraded printing? Rather than having labyrinthine steps to overcome out of toner error (which are not in the manual mind you). My experience is that full color printouts do not suffer. When they do run out, it's noticeable and comes nearly at once --rather than slowly degrading.