The only reason you would ever be worried about receiving "bad" coins would be because of some legal framework that makes such coins possibility "bad". The exact same legal framework could make 100% of Monero's coins "bad". And you're back to square one.
Having to spend contraband coins in a public blockchain would feel awkward, a private chain suits the use case much better.
Bitcoin plus this (or any number of other extensions)
enables Bitcoin to be
used in a completely private manner. I don't see any difference between Bitcoin with these easy extensions in how it is used, and Monero.
The difference is mixing. Bitcoin doesn't have mixing without using some sort of third party mixing process, and that payment code proposal doesn't provide one either. The coins remain traceable on the blockchain.