I think it serves only to hold up the plastic as you have supposed. Or maybe a sort of "limit stroke" for the rubber when the ball hits.
I had your same problem... I polished the nails together with the metallic parts. I decided to leave them "shining" inside rather than have a hole on the playfield. Of course the hole can be easily filled by a toothpick with glue, then retouched with colour... but for me is not the best solution in this cases...
Ciao