I'm not sure if there is some fine print in the rules that says you actually have to purchase the ticket w the Signature card ...
I booked PDX-SEA-(overnight)-IAD last month for travel last week, but did NOT purchase the ticket w the Signature card ... however, going PDX-SEA I had a couple bottles of wine in the bag, so had to check it at PDX; rather than going standby, I used the Signature card to pay the $25 to SDC to the next available flight, and was also charged the $25 bag fee ... in the SEA baggage claim area I saw a sign advertising the free bag for Signature card holders; spoke with a supervisor in the morning before leaving for IAD, and was able to get the bag fee refunded