StuartForrest, the reason the regs deal with the issue of low hour aging engines is because of the corrosion and accelerated wear that can occur inside an internal combustion engine that sits for long periods between runs. This is due to the fact that moisture and other contaminants can get inside an engine and begin to corrode it, when it sits for long periods without being run. This happens with autos and trucks and boats as well as airplane engines.
You mentioned that your engine is 12 years old and has only 600 hours, and it just had a top end overhaul with 0 hours since then. Think about why that top end overhaul was required? What is the condition of the rest of the engine?
I do think however that a sensible way to deal with this in the regs, is for an older low time engine to be inspected at set periods, to make sure it is not suffering from any corrosion problems. If an engine is found to be OK upon inspection, then it should be allowed to remain in service.