We had a plastic top style protecter when my DS was toilet training. Rather than washing it every day I would spray it down with disinfectant and give it a good wipe. Would get washed every few days. In a hospital (I'm a nurse) all the mattresses have a plastic/rubbery cover and between patients/if soiled, it will get wiped down with disinfectant..I don't see the difference?
But if it is a fabric style protecter you couldn't wipe it as the urine would absorb..that would get pretty stinky!