Hey all...
First, I have no problem with the "rebuild" being done every other year. Make my vote a "B". O-rings on nice polished metal surfaces can probably last five years or more. If they would all use Viton O-rings, I believe that contention would be ultra conservative.
O-rings are destroyed by grit, grunge, sand and shells ie neglect. If you keep your regulator clean, and soak/clear it after your dives (which is what you should be doing) you should have minimal problems with that.
However, "The Yearly Service" should not be just a mere inspection for leaks. They should doing a full performance check out including cracking pressure and going over all of the adjustments. Sometimes things fail gradually, and when you are breathing on it, you may not notice the breathing getting harder a smidge at a time. A competent, concientious technician will find that problem easily and can determine just how "deep" they should go into the reg. When you get your regs back, you just might notice that it breathes a lot easier, without ever having noticed the prior lack of performance before.