Member Sites would like to have something like Passport because they believe that End Users hate registration forms.
Supporting multiple reference APIs would be a pain in the butt for most Member Sites. If a single API allowed for multiple Reference Sites, that would probably be fine (i.e., no extra work).
A site that could be this and one of the Reference Sites would be Yahoo.
Ranked requirements for this group (for a Distributed Membership System):
minimize work to write/use code to support system (includes: no changes necessary to data structures, field/variable names, internal userID structure, etc.)
minimize variations to handle (mimic Passport API?)
minimize change requirements caused by this system (stable API)
minimize confusion to bonehead users to avoid turning them away
flag fields as optional or required (to reference system, for user approval, so user can withhold info more easily)
increase confirmation that user is "real" (reduce credit card fraud)
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