While i will agree that the client being able to validate the certificate directly is the best place to be, I do not see any advantage of requiring purchased certificates over self-signed certificates. IMO it provides no realistic security benefit at all.
Then again I don't award points for
certificate verification having anything to do with identity verification of the remote party.
In other words, if I didn't sign it then the certificate posseses no more validity than an ephemeral self-signed certificate.
Of course, others are free to delude themselves with additional "theatrics" and false assumtions if they want to do so.