You wouldn't have to buy in USD... you could transfer bitcoin (or whatever your pleasure is) and sell to USD on the exchange. They don't even need to let you transfer it out necessarily, they just need to give you the ability to hold it (like tether but with actual dollars). I'm not sure it will make too much difference, since it doesn't introduce new liquidity (most investors aren't adding USD and just going from crypto to crypto). It would just wipe tether out.