If rated battlegrounds are not going to have automatic group creation, I think that will hurt them as transient content. But what if Blizzard pushed rated battlegrounds towards the extended content side of the spectrum?
Instead of rotating the three brackets (10v10, 15v15, and 25v25) every week, making assembling teams very hard, what if each season focused on a specific bracket?
For example, PvP Season 9 could focus on 10v10, and the rated battlegrounds would be Warsong Gulch, Battle For Gilneas, and Twin Peaks. PvP Season 10 could focus on 25v25 and the rated battlegrounds would be Alterac Valley and Isle of Conquest. PvP Season 11 would focus on 15v15, with the rated battlegrounds being Arathi Basin, Eye of the Storm, and Strand of the Ancients.
The big advantage of this is that you could form your team at the start of the season and play with the same people for the entire season. You would only need to find a new team after the season ends, which is a natural point of change in many cases.
I think this change would encourage team play and mitigate the frustration involved in assembling a PuG group each week. You would win or lose with your team, and if you did well, your entire team would be rewarded at the end of the season.
Meanwhile, if you didn't want to form a permanent team, you would PuG a team each week, just like you would in the current design. You don't really lose anything under this system.
I think this might even lead to better gameplay, as focusing on a few specific battlegrounds for longer period of time might lead to more strategies and counter-strategies being created. You wouldn't need to master all 8 battlegrounds at once, you would focus on two or three at a time.
And the other battlegrounds would still be available as unrated options if you wanted to indulge. Blizzard could even remove the current rated battlegrounds from the unrated pool to keep things different.