Frankly I have always thought that accreditation was binary. More specifically, I have always operated from the model that TLDs really are unique, and, though certain TLDs may have similar policies, in the long run there will be many TLDs with many different policies. Hence it is quite likely that some registrars would not want to register in all TLDs. Consider, for example, .museum. These positions represent a stable, relatively constant, outlay of funds.
Registrars differ vastly in the load they put on that staff.
One could make other arguments, as well. Apportioning the cost to the various registrars, in other words, is not a simple problem.