Depends what you mean by use the facilities. Book and pay for a spa appointment, or buy a drink in the bar - fine, why not. I don't think hotel business models generally work on the basis that room rates subsidise those sorts of things, so you're paying the going rate, which is fair. Plus most hotels are quite happy for you to do that, IME.
Use the pool and the free towels, or the lunch buffet - not fine, you haven't paid for it. Those sorts of things are often monitored anyway.