I ususally only leave my homeade sillys out in PA, still have pick a few up at the end of the season around the fencerows, etc. In Md. we use stakes on the shells, tires, and silhouettes, they usually don't go anywhere, just have to stand a few back up each time you get there to hunt.
As for the wear and tear, your can hurt tires, the shells get fresh white about every 3rd year, silo's get the white touched up each year, not really because they need it, but I like my silo's to have very bright white cheek and butt patches on them, just personal preferance. Not sure what leaving "pretty" decoys out all year would do to them. But the ones I leave typically leave out, can really take a beating.
PS to Goose, I think they have already figured out the layouts. When those thing first came out, you didn't even need to brush them up, just throw them on the ground, hard to get away with that now.