Including bushings in a Motion Analysis study is equivalent to adding a flexible mate. You can think of a bushing as a spring and damper system containing a little bit of slop.
The SOLIDWORKS Motion solver treats parts as infinitely stiff. Consequently, you can place only one constraint (mate) restricting a specific degree of freedom on a part. In situations where more than one constraint restricts a specific degree of freedom, all additional constraints are redundant.
You can use bushings instead of mates and set the stiffness to constrain the motion along the direction of a particular degree of freedom. The load on each bushing is a function of the stiffness value.
Using bushings to represent mates and part stiffness, you can incorporate stiffness in Motion Analysis studies. The more accurately you know the stiffness, the better your results.