P2p exchange is great, but it has its disadvantage: you need to find a peer to make exchange, which can take time. Gateway service like openledger fills this gap. To make a deposit/withdrawal, you don't need to look for a peer, you deposit BTC to their wallet and few seconds later funds are in your bitshares account, but you need to trust their IOU.
Implementing a trustless p2p exchange in bitshares DEX, however, would be absolutely great. A company like openledger could become a mega-peer and make the market.