Just besides all possible metal-to metal noise and excessive wear and what more...
The chassis of the car has a sort of cup to hold the spring in a spring pad. If you leave out the spring pad, the spring will be able to shove in the chassis' cup. The spring pad is L-shaped and the thickness that is mentioned for the different spring pads is the part of the L that is situated between the spring and the bottom (actually top) of the cup. The standing part of the L has the same thickness for all different pads and is situated between the first inch of the vertical spring and the vertical part of the cup.
If you leave that part of the rubber out, the spring will be able to move in its cup (holder)...you can imagine what that will do to the handling of the car, wear of the springs and noise of the metal.
greetingz from a 8mm padded Benz driver,
with a minor 600+k