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It's hard to enforce on-time-use and revocation for self-validating tickets. That is, once you validate a service ticket it's not supposed to be re-validatable, to prevent replay attacks.
Potentially large content for ticket granting cookies
The content borne by a ticket granting ticket in CAS 3 is more than merely a String username. The ticket validation response can include arbitrary attributes. All of this content would need to be signed or encrypted into the ticket granting cookie for this self-validating ticket-granting-cookie scheme to work. Which might be fine – presumably a Cookie could accomodate this.