My philosophy towards proxies is: cui bono?
I am totally sympathetic with @Brass-Man's concerns about playtest card quality and can get behind rules restricting the quality of such cards. But I have not heard any arguments for why disallowing well-made proxies benefits either existing or potentially new Vintage players that I find personally compelling. A few points:
It's great that the hunt for power has driven some players to become invested in our community. But I don't see the existence of one path to joining the Vintage community as necessitating closing off all other paths.
"WotC will kill Vintage if it becomes a proxy format": this argument is odd to me for several reasons. First, (paper) Vintage is already de facto a proxy format, in that the vast majority of paper tournaments allow playtest cards in some form, and this fact has been acknowledged by WotC in their official communications. Second, WotC support for the format is infinitesimal. Other than a single tournament per year, (paper) Vintage is a community-run format.
We shouldn't reward counterfeiters or encourage players to give them business. This is a reason to allow playtest cards at Vintage tournaments, since counterfeits are already de facto allowed (are you really going to whip out a loupe and take your opponent's power out of its sleeve in the middle of a tournament, to police authenticity?)
Some people are concerned that allowing more playtest cards in tournaments would decrease the value of their investments in Vintage cards. While there is an undeniable link between card playability and value, all of the evidence I've seen points to very old cards holding value due mostly to (a) their collectibility and (b) their usefulness in other formats like Legacy and EDH, and only slightly due to their value as a Vintage game piece. Consider (1) the value of alpha and beta cards have skyrocketed, despite a stagnation in the number of (paper) Vintage players, and Vintage become de facto a proxy format; and (2) the high price of iconic cards like Shivan Dragon that see no Vintage play.