I read most of this thread but not all admittedly. But it would seem to me that one of the reasons for the mounts to be in this package is that The Keep was a level 1-3 module. So I am assuming that this will be a low level adventure pack with a legendary addition and not the level 10-17 that we have seen recently. At level 1-3 your movement speed is seriously restricted and IIRC the Borderlands map is a fairly large map. Running around with very little speed boost in a large map would seriously suck...hence, the additional need for mounts.
I'll also restate what has already been said. You guys who are complaining about bug fixes, etc taking priority over this are assuming that the same people work on both new material and fixing old...that's almost certainly not the case. It may seem that the emphasis is on new material but that could be due to that fact that it is probably easier to create new material on an existing code than it is to reverse engineer code that the devs have repeatedly told us is "spaghetti code".