Seeing as (allegedly) the main reason for passports is to stop drugged animals going into the food chain, then it seems pretty irelevant whether the animal is castrated or not.
It also appears to me a licence to print money to for the issuing authorities to charge for change of ownership. They charge enough in the first place for a passport, and what does it matter who owns the animal, if it has a valid passport that ties up with its microchip number.
There's far more cattle go into the human food chain than horses and the records and passports are managed much more efficiently with no cost to the owner. It's about time the whole thing was sorted out with one offical issuing authority. Anyone wanting to register a breed can still do so with the relevant society, like we do with cattle. But why should the rest of us subsidise breed societies etc that are of no relevance to our particular equine?
(I have a mule with a Richard Steele passport and if I want to register her change of ownership I have to do it with the Lippazaner Society?!)