For a good example of how this may actually work, read the NTSB report on American Trans Air 406, May 12, 1996.
The mask needs to work for two cases: depressurization and smoke in the cockpit. Several years ago, my MD-80 started filling up with smoke from the AC system at FL 370. That is not the time to find out that the mask is not properly adjusted, the hose is tangled, or the mask mike doesn't work.
I preflight the mask every time I get into a new airplane. It is the old quick-donning mask, so the headbands need adjusting, etc. I routinely (probably 3 times a year) find a mask with an inoperative mask mike.
I can't speak to the full face masks, but the Eros mask we had on the 757/767 was easily preflighted in the box. However, we are required to use it above 250, and I used to find them occasionally tangled or packed in a way that would be a problem if you needed the thing quickly. I found that they were very easy to replace once you practiced it...which, if you use it regularly, you can do.
One of the advantages of using them regularly is that you will be that much more comfortable with donning, setting up comm, and flying that way, so that the day you really, really need to do that you won't be swimming in glue.
That said, I also distinctly remember coming back up from my crew break approaching Land's End to find the O2 pressure on the status page reading zero. It only took me a couple of nanoseconds to realize that the reason for that was that I had left the emergency lever on when I preflighted it back at JFK, and we probably hadn't had any O2 since we were about number 4 for takeoff. That could be avoided by not preflighting the mask, of course, but you could also preflight it carefully and correctly. Fortunately, I am not a skydiver.
I also discovered, fortunately in the sim, that the smoke goggles provided by a particular company did not fit over my drugstore readers. Another useful check to make once in a while.
Bottom line is you're gonna get one shot at getting it right on the day you need it. Whether you are required to use it above 250, 350 or not at all, always preflighting it and periodically using it can't do anything but make your day go better when that moment springs...