I never depend upon any parking brake. I once did in a C 310, and was very alarmed to look up and see the plane slowly moving. The parking brake is a design requirement, and therefore, provided with the aircraft, whether affective or not. Few I have ever used worked in a way which would make me want to rely on it. If they do hold at low power, they still might not at high power, which can lead to a really nasty surprise at the worst time during a run-up!
I believe that "setting" the parking brake, and then going on with other actions, without monitoring the aircraft for motion, is very foolish. It also defeats the intent of "being at the controls" while the engine is running. If the aircraft is creeping forward on a slipping parking brake, and you the pilot don't stop it, you are responsible for anything going wrong - you can't blame a poor parking brake. Use your feet.
I respect the dilemma of being told to not remove anything from the manufacturer's checklist, while creating your own. I suggest that the phrase "Apply brakes" or "Insert wheel chocks" as appropriate be used.
Yes, Cessna parking brakes will become less effective with use. They can also bleed down, so they are not at all suitable for any length of time. Their use will cause strain on a "loc-o-seal" component in the master cylinder, which will eventually decrease pedal brake effectiveness. If you leave a parking break on, and someone moves the plane, something could get damaged. If it is applied, and then does not release fully (which certainly happens) and goes un-noticed, you'll get a longer takeoff run, and lots of brake wear/damage. If you get airborne that way, and then land again, it's even worse.
So I never use them (except to test at annual inspection time). Why are they there? To satisfy a design requirement, not an operational one....