If you catch the right moment, price difference on other exchange and BlockTrades can cover even those 7% fees... doesn't happen often enough.
They offer good volume, sometimes if you want to sell huge chunk of STEEM or BitShares and you don't have a verified account on Bittrex, Poloniex, you don't really have other options... STEEM is always disabled on Shapeshift...
Volume on decentralized exchanges is picking up slowly, so future looks good.
RE: NEVER use blocktrades, they are COMPLETE RIPOFF! 🤔