And they weren't, at least not Prot Paladins. They were subpar, but still usable if you weren't concerned about getting high ratings. With this change they won't even be worth the effort.
I believe the intended "niche" in PvP for tanks is base holding / flag carrying in (rated) bgs. Which they still excell at. They have no place in arena. Remember they moving in that direction also in MoP with druid feral being separated from guardian spec (aka not having anymore high damage in a tank spec, nor high survivability in a dps spec)
While I do agree this is going to hurt Tankadins in the FC'ing role, I do agree with Blizzard in this nerf. Blood DKs and well played Prot Warrs are just stupid...I generally just go the other way.
Anecdote is anecdote, but a Blood DK was holding the DMF arena against 4 other people (one of them a healer.) This will also help against easily achieved OP "tanks" in low level BGs, since it'll be far more difficult to 1-shot someone with Avenger's Shield...