The passport is NOT required for CASS verification by any TSA SD. IT WAS required for a period of 1 year after CASS was officially allowed. After that 1 year period TSA removed the passport requirement since pictures were now required.

However, some airlines have not removed the passport requirement from their FOM (GOM, OM etc) either as an added security requirement, or because they just haven't got around to it. Since it is hit or miss as to those that do (SWA, Delta), or those that do not (UAL, AA), it is just better to have it with you.