It is unfair to judge Proxy based on what came before, just as it was with Urzas Command to Cryptic Command, but only in the newer formats
Like tax cards, this wont win you any friends, but it might just win you the game
Rhys the Redeemed can double any tokens you already control
Swiftfoot Boots can protect Purphoros or all-stars like Krenko, and Ashnod's Altar is a fine way to sacrifice tokens for extra mana for those costly curve-toppers
It's a mana sink, and if your dork brings mana with it, like Priest of Gix, then you can make infinite blinks and then many more combos too
Since many Vehicles are colorless, the mono-color restriction is hardly a downside