We have a travel cot and I bought an extra mattress to go in it. This was mainly because Grace slept in the travel cot in our room when she outgrew her moses basket so I wanted it to be comfy.
When we went away before Christmas, there was a travel cot in the cottage we stayed at and it didn't have an extra mattress. She didn't seem bothered and slept in it fine.
Personally, if you don't mind spending the extra cash for occasional use, I'd get the mattress just for comfort, but the cot is perfectly useable without one
HTH