I've never traded to USD. Maybe it's good for short term tradings - like a one day trading or so, when you buy some altcoins using X sum and selling when the price go up, getting 1.3X for example. Then you get your profit in USD immidiately. But if you don't need your money back immediately, for long term perspective I think it's better to trade to BTC, as it will always increase in price. In that case getting more BTC while trading altcoins, is good.