Perhaps my understanding is outdated, but I remember a time when the GPU was somewhat limited when virtualized since it potentially needed to be shared as well be exposed through a common layer.
Maybe this doesn't matter for all use cases, but for Plex especially when it's doing transcoding, I'd imagine it's very important.
The challenge is getting your friends and family to change the default prefer-transcoding behavior every time they fire up a new client.
I know you can use tricks like tautulli events to terminate transcode streams but they're difficult to deploy and a bad UX for the users even at their best. Better to have Plex tell users something like "This content can't be played at your preferred bitrate X, would you like to use more data to play at full bitrate Y? Yes/No/Always"
On the other hand, I was able to very easily get GPU pass through working on Emby inside and LXC container.