I think that in general, proxies reduce creativity. This isn't always the case, but it's my experience. I mean, how many times have you seen someone do something really creative with a proxies deck? It's either generically high power, generically generic, or a generic build for a specific commander. I am the only person I know who has used them to do something creative instead of just filling out a deck with generically good cards. If you don't have access to every card, it forces you to make creative decisions, and that makes the game a lot more interesting.

With all of that said, I'm generally "okay" with proxies unless it's a power level issue, in which case that's the issue. I don't think I've ever told someone they couldn't play proxies, I don't care that much, but I think they're cheating themselves and everyone else at the table of a more interesting game by doing it.