In the sense of using arena data as a factor in balance.
As far as I'm aware, the vast majority of PvP that takes place in WoW is in random BGs - since this is where the most people play, this should be the focus for your game. I'm sure the hardcore arena players will start screaming at me now, but they need to accept that they're an increasingly small niche, and I don't see that changing.
To me, this means that rated Battlegrounds should be where Blizzard pulls the majority of their data from for use in making balance decisions; arena does not translate well to the random Battleground environment, while rated Battlegrounds do.
A lot of the things that make people get frustrated with PvP (which, again, for most people is random BGs) exist solely because of the emphasis placed on 3v3 arena as a balancing factor. The first things that come to mind are healers being extremely difficult to kill, and the sheer amount of disables in the game. Kills in arena games are often made through forcing cooldowns with disables, and then doing it again once DR timers have reset to secure the kill. Likewise, healers in arenas must be very hard to kill, otherwise there would be little reason to bring a healer along.
Because BGs allow unlimited respawns and include more players (which also means more classes available), this results in a lot of potential silliness that can't really go away unless they want to also separate arenas and BGs... which would require a lot of work and be pretty dumb.
So, again - why are Blizzard using 3v3 arena as a balance focus when, since the vast majority of players just play random BGs and little else, rated BGs are a much more relevant source to draw data from?