Flex mode raiding was created because there were people for whom LFR was simply too easy/disorganized/unchallenging, but normal+ was too difficult/time consuming. Flex was designed to be the middle ground, and successfully created a place where 'average' players could raid in a setting appropriate for them.
Battlegrounds only have two modes: rated & random (essentially Mythic & LFR in comparison). I'm sure I'm not alone when I say that I find RBG's too serious for my tastes, but randoms aren't serious enough and are often too disorganized. Yes, you can make premades but there's no fun in plowing over a team of randoms, and they can often be hard to find anyway. War games let you fight against another premade group but don't offer any rewards or incentives.
What I think we need is a middle-ground mode for Battlegrounds, something between rated and random. Such a thing might be hard to create, but I'd like to see it happen. I'd like a premade-only mode that pits you against other premades randomly selected from a queue; a BG that offers more rewards (honor & conquest) than randoms, but doesn't award any rating or RGB rewards (no rank titles or mounts etc).
Random BGs should be for 'casual' players and gearing up,. Premades for the 'average' player. RBG's for the serious/dedicated PVP'er -- such a system would have something for everybody, allowing everyone to play with groups of their own skill & gear.