I normally use fidelity AmEx or any other cards that gives 2% cash back.

Because say you spent $40k using SPG. Then at AA, you'll get 50k miles. Using other 2% cash back, you'll get $800.

800/50k = about $.016 or 1.6 cents/mile. Personally, I dont like to spend anything more than 1cents/mile, so cash back was better deal.

Recently there was a AA promo during Christmas where you got close to .25 cents/mile, hence proving my point.