No one can stream with Blu-Ray quality yet. Consider most blu-ray encoding bitrates are at between 25-45Mbps. Netflix's best HD Streams I think are about 3Mbps, maybe slightly higher. Even if you're streaming at 1080p (which Roku now supports) that stream is going to be smashed to bits to get that resolution out of it.

Also no one is streaming 5.1 surround sound yet either. You'd chew up your whole bandwidth allotment on audio.

Blu-Ray is still a considerably better format for video then streaming.