I agree with you about the cost. It is more about the limitation of choices that can do the horizontal thing. It is an esoteric feature but limiting in the choice of equipment.
I think if you polled photographers, the amount that use this feature - and then the percentage of time that they use this feature - would be small. There are applications - such as macro (in cases) where it applies. In these, is as much about controlling a razor thin DOF as it is about vibration reduction. Even then, it is necessary to use considerable technique to isolate vibrations etc...
Additionally, it really has severe limitations in terms the physics of the situation. The main purpose of tripods is for vibration reduction. You give most - if not all - of that away by the time that you cantilever your camera out on the end of a lever arm that can have the effect of actually amplifying the vibration. Once you go past 150um or more of vibration over the shutter time, you are in the realm of handholding. So for this, if you use a lens with IS and you do this to control DOF, then it is usable. As vibration reduction? No.
For that reason, I'd consider this capability bonus points if you decide on a tripod you like for other reasons has this feature. I would not recommend seeking this feature as a prime reason for which to buy a tripod for the reasons above - it looks better than it is.