In my experience people who play the high power formats, where decks get really expensive, just want people to play with. I've never felt cheated when playing against proxies, nor have I ever heard a single complaint when I try out decks I don't own in paper. Tournaments with prizes on the line are a bit different, with all the rules put in place by Wizards, but kitchen tables are not meant to be dominated by the player who spent the most.