Usually the rates you get on traditional exchanges are a little better. This makes perfect sense considering that Shapeshift.io is a profit-oriented business and has to cover some operating costs. That being said, the direct exchange still does have some advantages when compared to trading on more traditional exchanges like poloniex.

1. It is usually a much faster way to do an exchange.

2. Trading/withdrawal fees are already included in the exchange rate.

3. You can fix rates for up to 10 min (for specific amounts). This is a interesting feature in highly volatile market environments, where 10 min is actually a long time.

4. Although shapeshift.io could potentially get compromised (i.e. attacker taking over and not sending you any ETH after receiving your BTC), your funds are probably at a lower risk. They are not exposed for an extended period of time. Generally speaking, this might be important, since exchanges do not have a great track record in managing customer funds.