You don't need the withdrawal step in the current market
I buy BTC w/ USD
Bitfinex prints USDT
Bitfinex uses USDT to get BTC
In the end of this operation bitfinex has the USD and everyone else is satisfied. Normally Bitfinex needs to hold onto it to settle USDT claims, but so long as there's a USDT-usable market this might not be necessary
Even if USDT is at 50 cents to the dollar it's still a license to print money
For every buyer there is a seller, bitfinex are not selling you the BTC. So that seller now wants to trade his USDT for USD to withdraw, whether he does that through bitfinex or another exchange using USDT, the end result should be an outflow of USD and a corresponding drop in the number of USDT.
1. For convenience, like if they're speculating in BTCUSD and their exchange uses Tether.
2. Deliberately, because they're breaking the law and they'd get caught if they tried to deposit their money in a regulated institution.
I doubt many people in the first category have thought much about the risks, or even know the difference between USD and USDT--if they had, then they'd probably use a different exchange. I'll bet there's a lot of them, though. I could imagine that people in the second category would consider the risk and still want USDT. A portfolio of half each BTC and USDT is probably less risky in some sense than all BTC. Even if the investment in USDT has negative alpha, it might improve your portfolio's Sharpe ratio, if the correlation between the risk that USDT collapses and the price of Bitcoin is small enough.
I buy BTC w/ USD Bitfinex prints USDT Bitfinex uses USDT to get BTC
In the end of this operation bitfinex has the USD and everyone else is satisfied. Normally Bitfinex needs to hold onto it to settle USDT claims, but so long as there's a USDT-usable market this might not be necessary
Even if USDT is at 50 cents to the dollar it's still a license to print money