Considdering that Plex Server is already cutting the video into chunks and serve them to the client, it wouldn't be such hard task to monitor the flow and correct the bandwidth to the client.
The benefit would be that clients with poor internet would always get the best available quality - albeit varying through the movie. shifting between 3mbit 720p and 4mbit 720p is not noticable.. and shifting from 4mbit 720p to 8mbit 1080p would just crisp the image slightly.
At the current state, i have 2 clients that have had to lower the quality to 3 mbit just to be sure that the video wouldn't just suddently stop in an endless buffering - although they have 10+ mbit internet !
I guess the player has to support it - in effect all of them ?
if they support VBR already it should be a problem sending 2-10 mbit 1080p video without switching streams ?
anyhoo - IT WOULD BE AWESOME !