Warning (Added 2015) Some of the documents that we previously received through FOIA suggested that all major manufacturers of color laser printers entered a secret agreement with governments to ensure that the output of those printers is forensically traceable. Although we still don't know if this...
This is why they won’t let you print black and white without cyan or yellow!?
Yes, assuming your printer has a black cartridge. (Otherwise, it’s because it legitimately needs all the colors to reconstruct a shitty black – I don’t know if they still make printers like that, though.)
Using C,M,Y in addition to black makes “rich black” in printing applications. Without the K or black component the best you can do is a dark brown.
You don’t need to use additional colors other than K for black, but they do make it a deeper more rick black.
Really only applies when you are printing photographs or high quality images. For text, its a rip off the uses more ink.