Im not saying bgs cant be serious pvp, look at the rated bg scene in cata and early MOP, but random bgs as such have never been "serious" pvp since arenas came out, in a random BG 99% is decided by gear, sure the 1% can be great but they happen very rarely. And even then, without rating, there is no real downside up upside to winning or losing a BG bar honour gain - which once geared and reasonably rich is sort of irrelevant.
In arenas skill is what matters, not what gear or class people are, or how much healers one side has. Hence it being the serious side of pvp since forever. When was the last time there was a bg or rbg tournament?
And then there is this weird bg culture here, where while people take bgs seriously they totally suck at them, which confuses me a bit. Eots and arathi in particular, people are endlessly going for 4 and 5 caps. No one calling incs or defending properly. This has the almost comical side effect of 60% of a team riding around the bg in circles capping base after base (which get recapped 5 seconds after they are gone, cause no one stays) while 60% of the enemy team does the same thing, just a few bases behind so that the 2 teams almost never meet.