I'm going to weigh in on this, despite the fact that I don't actually play EDH, so take my opinion with a very large grain of salt. My understanding of the format, is that it was never meant to be competitive. It was meant to be a casual format. What is different between EDH, and any other constructed format? 1) Larger decks, 101 vs 60. 2.) Singletons, vs. up to 4-ofs. What does all this equate to? 80 something unique cards, vs maybe 20 at most. The combination of these things to me, says the spirit of the format was that it was meant to be a collectors' format. It was about what unique cards you could collect to make up your 101. It was never about power level, as it was never intended to be competitive. (With 101 cards, and mostly singletons, the variance is just too high to ever be truly competitive IMO.) As such, proxying the powerful cards that you can't afford is antithetical to the spirit of the format to me.
Where I could see justifying it, is if one person in the group has collected a handful of high power level cards... You can't really have fun playing with them since they're playing on a higher power level, and it's not fair to ask them to not play a prized card that they probably had to work decently hard to acquire.
All that said, to each his/her own. If your group is all ok with proxies, have at it. For me though, I would much rather have everyone play at a lower power level with cards they can reasonably acquire, than jack the power level up by allowing proxies, even if that means one or two players have to sideline some of their most powerful cards.