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...
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.