Worst first post ever.
Casascius has been around since just about the dawn of Bitcoin. AFAIK, there have been exactly 0 accusations of him keeping and using coin privkeys (keeping in mind Goat, one of the most temperamental Bitcoin users of significant public awareness, has had thousands of bitcoins in Cas coins). There have been no cases I'm aware of where the seal's been successfully bypassed and sold as legitimate. Unlike self-printed paper wallets, Cas coins can be traded in a relatively secure manner offline because Casascius is a trusted third-party.
Worst 798th post ever. The OP is not accusing Caucasus of stealing, he says people shouldn't trust physical coins. Even if you buy a Caucasus coin from someone, the seller could have peeked behind the seal, using the right equipment, especially if BTC price is high.