They've already said that tanks getting mounts are there as incentive to have more tanks. This game, and any MMO, suffer from a severe lack of tanks. The tank mounts are meant to get people who normally wouldn't tank, to start tanking some higher tiered content. Even if people have these mounts and will never touch tanks again, it would mean that the mounts have already served part of the purpose by having these non-tanks to tank 200 instances. That's really nothing to take lightly.

As for the DRK mount, people already know what it is. Hints were dropped months ago back at Fan Fest even. They're getting the Batmobile.