I'd imagine the a large portion of the players who would normally be queuing for battlegrounds will instead queue for rated battlegrounds.
In the long run, I don't see rated battlegrounds as that much more difficult then normal battlegrounds.
There will be a lot of PUG quality players in battlegrounds, without ventrilo coordination, who just want to kill shit.

There will be premades that only run together, and run at specific times the same as raids.
And these will be rather challenging teams to play against.

Still, there should be a larger overall pool of players in battlegrounds.
And the system will attempt to match us against the team closest to an even match.