I do use a FF affinity card in my business, but try to keep it under control. You do not want to pay ANY interest on the card and you DO want to pay it off every single month.

I use the AA Mastercard for certain Vendors that accept it. Although usually only to help with the Cash Flow.

For Office Supplies, Costco accepts AE and I use my Starwood Amex for those.

Again I do pay everything off every month. The Finance Charges would offset any Miles that I would earn.