I'm still really new, but the concept of finding first of all, something hidden and maybe even some treasures is very exciting to me. Of the two caches I've found so far, neither cache had anything inside other than a log close to needing maintenance. Part of the thrill is the possibility of finding something cool so in one cache I put a quarter (fit the theme of the clue and size of the cache). In the other, I was scrounging for something good, but all I had was a baby's toy. A small treasure to make up for shuffling a badly deteriorated log or cleaning out any trash keeps the game fresh, however it is understandable that no real-world value usually goes into the caches that have a higher volume of muggle traffic. If I were to plant a very remote cache, I would have no hesitation putting five bucks in there as a prize for the FTF.
Consider that we are of a unique community of hobbyists that that already require a piece of somewhat expensive tech in order to participate. Factoring that, placing trash or foliage in a cache goes against the idea of the game.