The passport is required for CASS verification, unless you are on your own or an affiliated airline which uses an internal system for their employees (ie APOLLO at UA). You can legally cabin jumpseat without CASS, but some airlines may have a policy to use CASS to verify your eligibility for the cabin too. Also many CS agents just always use CASS and may not be aware that you can cabin jumpseat without it.

If you're non-reving or commuting, better carry the thing. It's not that large or bulky, and will save you hassles. Mine lives in my right front pocket, they are sturdy and will not wear out in ten years.