Ppl keep jumping from one BG to another when they are about to lose. The BG ends up with a completely unbalanced horde/alliance ratio which takes any challenge away and makes it a poor pvp experience.
Due to that problem Blizzard reworked the BG system after TBC and implemented RBG.
Since we have no RBG in TBC I suggest to disable BG switching. Players can still queue for multiple BG at a time, however when they enter a BG they have to play it until the end or get deserter in case they leave that BG.