In a Proof-of-Stake system similar to bitcoin, a large number of coins could lie dormant and accrue 'coin days'.   If these coins are in escrow, like on Mt.Gox, their 'coin days' could be used to do an attack on the network.

Thus any large escrow service would be a threat to the network, in addition to large miners.

Thus either escrow services must pay interest, or the need for escrow should be eliminated by a better block chain design and p2p exchanges.