A motocycle falling over , even at speed, imparts very little force. The force in a low side fall is pretty much just the weight of the bike falling over and impacting the ground. The direction of that force is the same even if the bike was doing 100 mph at the time. The problem is not the impact force, but other impact forces that occour after the fall. If the bike is sliding on flat pavement, all of the force is now directed about 90 degrees off from the original impact. If the slider hits a pothole or something that would impede its travel, the strength of the bolt attacthing it to the bike would be more crucual than what the slider is made from.If the bike hits something, like a fence or pole at this speed, damage is going to be extensive, and again, it would matter little what material the slider is made from. Now if the bike, as it slides, flips over, and impacts the other slider, or the bike is involved in a high side crash, so that the foward momentum of the bike can be factored into the impact force, you might be able to make a case for using a "softer" material when making the slider.
But just because it is plastic instead of metal does not mean it will not still impart enough force to damage something like the frame. Yes helmets are made to absorb impact, but please note that they can only do this up to a point. Helmet or not, if your head hits a wall at 100 mph, you gonna die. If you think a 3-1/2 inch piece of plastic is going to save you money in repairs over a 3-1/2 inch piece of aluminum in a severe crash, your welcome to try your luck. What I do know is if my bike is sliding down the street without me, I rather it be sliding on something I know won't fall apart.
KM