Broken mainsprings are not common, but I wouldn't call them rare, either. They are, after all, metal springs and therefore subject to high cycle fatigue. It happens. And yes, a scratch can damage the microstructure and start an eventually catastrophic crack.
Hand springs and trigger/bolt springs are metal springs, also. About the only improvement in the design in the last 100 years has been the wire trigger/bolt spring, which has longer life potential.
Cleaning: no, it's not necessary to do a complete teardown cleaning every time you shoot the gun. I agree that it's fun, but you have to consider that the teardown itself imposes some wear and tear. At some point the insurance costs more than the insured, and I think that's the case when you completely teardown and clean every time out.
I do complete teardown cleaning on a time basis, approximately every 4 months. That way some of the guns that don't get used as often get an inspection to see that the last cleaning and oiling was adequate.