People are sometimes connivers and thieves. Mostly I think we're just occasionally thoughtless. The end result is that Caches are going to degrade over time.
What happens when I get to a cache and realize I didn't bring swag as good as what is there already? What if I really wanted one of those cool items? I'd like to say I would always take the high road and TNLN, but it's possible I might rationalize a poor decision in that circumstance - human nature being what it is.
In hindsight, I realise the caches I have hidden have much better stuff than what I have left at other's Caches - but I don't think I have traded down either. Perhaps that's only because I usually take the much disparaged McToys for my 4 year old. She doesn't think they're crap. She thinks they're "treasure" Daddy hunted to get special for her and she displays them as such. I think she has the right idea.