I concur, the clear nylon cheap ones suck.
OTOH, there's also better ones that won't budge no matter how hard You tighten the screw. https://www.google.fi/search?q=rack+...&bih=605&dpr=1
I use the non-countersunk black cup washers that I salvaged from somewhere, never had a problem.
I use them only for preventing the paint/anodizing from marring on more valuable rack gear though, the ground loop prevention/isolation necessitates the rails to be isolated as well.
Tried that once, and IME lotsa work and expense for no benefit what so ever in budget MI/PA gear.
LAB gear = different story.