If a torrent is already well seeded then downloading in random order isn't really a problem because there are already multiple complete copies out there. If it isn't, then the streaming client will likely receive less data from non-streaming peers due to the data it can offer being less rare and desirable, given several peers in the swarm downloads in sequence. That makes them all even less likely to be able to stream without pausing for buffering when there's not already a lot of capacity. So it probably works itself out alright.