I've said this before, and will say it again. I don't believe that PGP docs are really a significant thing to consider. There's nothing wrong with them of course, put I dont believe that coins with them should be worth more. I dont believe there is anything suspicious about coins without PGP docs, and I dont really see the value in proving that the seller is the 'legitimate owner' of them, as transactions with escrow are pretty safe.
Do you mean that most coins without PGP docs are counterfeit/fake or compromised?
For the record, there is and has never been such a thing as a fake or compromised Casascius*.
*outside of a hackathon and a picture of stickers from an unkown source. the stickers were very badly done, and the 'hack' is in my view nothing to worry about.