Yes, those are good to have. For those that misunderstood the OP, he is referring to the hip belt buckle on backpacks, not the belt for your pants. They frequently break and spares are available.
If all else fails and you do not have one you can remove both buckles (male and female sides), which at that point will leave you with an open loop of strap on each side. You can then take a length of cord, pass it through them and simply tie both ends together like a shoelace. Not pretty, but will do the job and get you all the way to the end, or at least to a shoppe that sells replacement buckles.
I always carry a piece of nylon paracord about ten metres in length. It weighs so little it hardly registers on my small scale, and it can be used for a multiple of uses such as a field expedient buckle, shoe/bootlaces, laundry line, tie down waterproof cover on pack, pack repair etc. There is a reason why use see so much of it in the military. Just make sure you burn the ends with matches or a lighter after you cut them so as to prevent unraveling.