ALWAYS.

If you carry a Nalgene or Guyot, get one of the space saver cups or something larger, even if it is just a coffee can.

If you carry a USGI 1qt, get the cup.

If you carry a kit that is too big for your pocket, but too small to be all of a pack, put it in a mess tin or a coffee can. Or use that to protect things like your camera.

If you carry anything beyond what is in your pockets when you go in the woods, in my opinion it is unexcusable to NOT have a metal cup or pot with you. I can make fire without metal, if I really, really have to. I can make rope. But I can't make a good knife blade nor a metal pot- the cup is one of Raven's Top-15.