The devs have stated repeatedly in the past that PvP is one of the hardest parts of the game to test and receive feedback on before a patch/expansion goes live. Not many people do Battlegrounds or queue for the Arena on the test realms. This looks like an attempt to get as much testing as possible in a somewhat-controlled environment, where normally they wouldn't get much useful data at all due to simple lack of participation.
As Zarhym stated last night, it's easier to observe how each of the classes are handling themselves when it's in a fairly simple, last-man-standing 5v5 (or whichever bracket) environment rather than the back and forth of Warsong Gulch or zerg of Alterac Valley (my words, not his).
PvPers, this is your chance to nerf Death Kni-err, have a hand in making sure PvP is as balanced as possible before Patch 3.1 hits live realms. Hop to it! You know, if you feel like it. Again, this is happening on March 26th.