You don't have to add it unless you would choose to add it.
I shared your opinion, in the beginning, but was forced to re-examine my prejudices by a couple of specific examples where the individual was in a situation where he wished to reholster while both seated and belted. Certainly, if one could make the argument that - if you stopped and unbelted and stood up and took care to ensure that the muzzle did not cover you while reholstering - the 'gadget' could be superfluous. In a circumstance where one could not (stop, or unbelt, or stand up), and assuming that reholstering was something considered necessary, the 'gadget' might offer some value.