If you can't monetize them without spending them, then they just don't seem worth it.

As mentioned by someone else, you lose too much opportunity cost; cash in bank, and reward bonus for just making the purchase with the CC. I would never use them at the 'pump' because I'm grandfathered into the that nice 'Chase Cash Plus Rewards' program that gives 5% cashback at gas stations, drug stores and supermarkets.

You're likely better off buying travelers checks with your Amex at $1000 per week.

Naturally, if you find an easy way to turn these back into cash, please do share the information