I had some trouble with a particular tree shedding sticky seed husks onto one of my tub covers once and made a cover like this out of a fitted bed sheet. But I only used it during the shedding season. My guess would be yes, condensation could be a problem with one of these waterproof ones.
Not that it is necessarily needed full time. Does it increase the longevity of a cover enough to offset the 70 bucks? Maybe.