So what is the purpose of a neck plate cushion or gasket?

[Silvertone Jockey]

and how might a cushion change the sound or sustain when used if at all?

[dmartn149]

I've been wondering the same thing for the longest time. Have you noticed that they seem to be mostly on Japanese guitars?

[Jazz P Bass]

My take on the neck plate cushion is it is simply that: a cushion.
1: it helps as a lock washer for the neck bolts
2: it protects the finish (from lazy ass manufacturers that do not deburr the plate properly before plating)

[in a vacuum]

As someone on the periphery of product and service marketing, I might conjecture that that three cents of plastic is there to provide an almost unconscious clue that that guitar is somehow better than the ones hanging on either side of it in the store that do not have that added feature.

[John_H]

They keep the neck plate from digging into soft body wood. I'm sure they have way less negative impact than a "Micro Tilt".

[Chuck H]

My old strat is just plate to wood and has bad denting at the corner areas of the plate. I've had under tightened necks shift during performances and I'd rather live with the cosmetic blemishes. I probably wouldn't have used a plastic gasket either 'cause, well, it's a plastic gasket! I have no GOOD explaination for why that's so unappealing, but it is.