The guy where I got my fronts foam filed said that the fronts, because they also are used for steering, tend to grind the glass into themselves. The back don't seem to do it.
Also thinking about it, I would guess that the fronts have a greater PSI between the tire and the ground, given that they are so much smaller than the rears. This must also help drive anything that is there waiting to do damage up into the rubber.
But now that my fronts are foam filled...well...that certainly gives me some peace of mind. I now still stop and pick out the most dangerous looking pieces of broken bottles though. I REALLY do not want to deal with a puncture in my loaded rear tires!
Bill