Agreed - airline credit cards, as compared to SPG Amex, are nearly all a bad deal in the long run. (There are occasional exceptions related to promotions or bonus elite-qualifying miles, but as a rule, airline credit cards are a bit weak.)

But...viewed simply as a sign-up bonus in exchange for one annual fee, they can be worth your while.

For example, a 20k upfront offer with a $75 annual fee is likely well worth it if you have a use for the miles. Just make sure you cancel that card within the first year.