Think of a coil spring as a long torsion bar that just happens to be coiled up.  When the spring is compressed or elongated, the "wire" rotates slightly.  The rubber is there to help keep that rotation motion from squeaking as the car goes up and down. 

Like Daze though, I never even knew those came on cars (I have been doing Mustangs since 1974) until the aftermarket started making spring perches not too many years ago.  The bottom line there is I never knew I needed them on any of my Mustangs (quite a few) that I owned before they started being reproduced, so I never missed them.  I think Ford did originally install them but they just did not last and fell out pretty quickly.  The new ones I have seen are so thin and flimsy I cannot imagine they will be there for very long either, but the good news is that it apparently does not matter.  There is so much force being applied over such a small area that they just don't stand much of a chance of surviving.  The upper pads will last alot longer because they have more surface area, thus less force in any one (or TWO as in the lowers) place.

I think a very small dab of bearing grease where the spring sits in the top of the perch will suffice to keep any rotational business from causing any harm.