fishinwrench has a point, even with regulated guns. With that said, there are other reasons as well. One reason you will see several air-gunners point out, is the stress and wear caused by high pressures. O-rings, seals, regulators, air ways, cylinders, etc. are subjected to the higher pressures and in some cases can fail or at least wear at a higher rate than if exposed to less pressures. There is the safety issue as well, higher the pressure the higher potential of a larger radius of damage to personal and property should a catastrophic failure occur. Hope this helps.
Me personally, I find all my regulated guns still have a sweet spot and it is always below maximum fill pressure.
