Why preflight per-resource rather than per-origin?

Despite the costs of doing preflight opt-in on a per-resource basis rather
than a per-origin basis, to meet its security goals, CORS proposes to do
preflight on a per-resource basis. I have seen the rationale for this stated
in bits and pieces. Can anyone point me at a reasonably self contained
statement for why we need preflight on a per-resource rather than a
per-origin basis? If there's nothing adequate to point at, could someone
state a reasonably self contained rationale for this? Thanks.

-- 
    Cheers,
    --MarkM