You again don't see it. These are not for the average quad. These are for those that have already shaved a lot of weight off there quad.
These are not for a 390lb zilla, these are for the sub 300lb hybrids and drag quads.
I know the nylon screws will provide enough clamping force to seal.

With any seal you use you need a screw that'll slightly pull up on them and I'll bet you that the plastic screw will strip the first go around, think of most rigs they use them on interiors or fender wells as a one thing use because any pressure they strip.