[Symmetry]

In practice force sensing is more useful than proprioception in most cases, at least for grasping. Generally you won't the dimensions of an object you're attempting to grasp anywhere near as well as you would know the shape of the hand you're using so a certain amount of underactuated compliance makes the job a lot easier.

[stefanka]

I agree. But even then elasticity had to be taken into account (and maybe even gravity), if the object is very delicate.