I would be very curious to know how often dive resorts service there rental gear? I've seen some
really scary looking regs, rented out.
Can only speak for the places I worked (and worked on the regs), but if you have American customers, or you are in America or its colonial possesions, you'd better have a log with the regs, their serial numbers and proof that you have overhauled them at least annually (per manufacturer's stated service interval). The liability is too great if you do not. Even though gear failure should never be the cause of an accident if you are actually 'following the rules' of recreational diving, insurance is expensive, and the umbrella policy writers usually come out and see what you are doing, and demand that sort of policy. (Or at least they should.)
Since we had no way to keep track of how many dives a given set did, that was the minimum: annually. Any gear that got sent back by the guides (for whatever reason) also got a thorough inspection, and often had the annual done right then and there.
Rental gear plastic looks bad because plastic does not wear well, and the gear tends to get dragged around, especially the octopus and console. And the chrome bits corrode. At least in the tropics, the air itself is salty enough to corrode the gear even if it is never used. And it's too expensive to keep gear stored in air-conditioned environments to prevent that.
There are tricks to keeping gear looking good that might actually make it less reliable, like soaking gear overnight in a very weak vinegar solution. It makes it sparkly, but probably eats on the o-rings. From the appearance, it looks better maintained than gear that is not soaked in vinegar nightly.
(The bit you really do not want to know about is the stuff you don't see: the vomit in the second stages. The real reason why instructors/guides use their own equipment: because that way they know no one puked in it yesterday. There is almost no way around that one. You cannot take apart every second stage every day to check for chunks, but if you don't.... At least masks are easier because they have no internal spaces so they can just soak them in bleach overnight. Soaking serves a second function in that it keeps the cockroches from nibbling on the silicone.