Well, As it stands, Having an Exp-off Wrath, Or cataclysm account puts you into the same bracket as those on a BC-only account, and vice versa.
All battlegrounds in the bracket are 70-74, including Alterac valley, which, as ranta says, tends to pop quite a bit.
As it stands, there isn't a glaring reason i can think of that would make a burning crusade account superior to a wotlk/cata account, but hey, upto you.