I'd like to be able to access joint positions as parameters so that they can be adjusted programmatically, or via an external expression.
Two examples of why this would be useful:
1) The alignment of a joint may depend on the dimensions of something that it's attached to, so being able to create an expression in the parameters window means that this will update automatically.
2) The dimension of a particular feature may depend on the position of a particular joint, so being able to have live updates on this would be very helpful.
An example of case (2) would be if one had defined a spring using the coil command and set the length to be the distance between two faces, which can move relative to one another. With the ability to access the joint position, you can adapt the overall length of the spring so that as the faces move, the length of the spring changes to reflect the movement. This will make all sorts of animations possible, and will allow you to check that there's sufficient space for the component to compress into.