it sounds a bit too idealistic. How about if someone is hired at a hire rate because "well we needed someone with his skill set ASAP, so we agreed to pay him a premium" (but we can't afford to pay everyone at that rate) or "We desperately needed someone quickly with skill set X and he was the only person available"

And sometimes comparing people and their skill sets is really apples to oranges. If one guy is an expert on some very specific top, and thats important for your business (and is therefore an expensive hire) - it doesn't mean you should create an incentive for other employees to learn his skill set (maybe you only need one statistician or expert in COBOL or whatever)

"There's no compensation system that makes everyone happy"

Sure, pay people way above market rate and don't allow them to compare wages. They will not feel ripped off and they don't develop a sense on inadequacy