Turning the main off all the time is probably not a good idea; I believe it would lead to air in the system and general inconvenience. Remember also that if the main pressure is turned off, safety and prudence would demand that gas or electric to water heater also be turned off.
There is a system available , I think it is called Water Cop , that monitors flow in the system , and by "learning" your usage, will shut off the main if it detects an unusually large flow, or flow for too long a time.
I understand your thinking, and actually have done some research on systems. BUT, keep in mind that something like 100 million buildings have water on all the time, and catastrophic failures are rather rare. The best peace of mind should come from having all work done by trained, licensed professionals.