Master valves are your last line of defense on a well. Without a reliable master valve, your well control is compromised, and that is never a good place to be. I have had the opportunity to work on a lot of old, mature wells, some which still have a lot of pressure, and it amazes me how many of them have master valves that are either leaky or extremely difficult to turn. If you were in trouble and had to close that valve quickly, you'd be out of luck.
Why does this happen? The answer is pretty straight-forward. Valves need regular maintenance just like any other mechanical device. It costs money to change the oil in your car, but you do it anyway because your car valuable and the cost of failure is large. Master valves are also extremely valuable. Take care of them. Keep your valves greased up and when they need repaired or replaced, spend the time and money to do it the right way. In the grand scheme of things, this is a small expense to keep your wells safely under control.