Essentially the additional cost of paying via credit card (mortgage+2.5-3%) yields a cost that is higher than the potential points earned when used. Using it on occasion such as to meet minimum spending on a card to get a signup bonus or to top off a points balance to reach a redemption goal can be worth it. I recently used my CIU to pay my rent to meet the minimum spending absorbing the cost of doing so (2.4% via my building's management company) as the signup bonus is worth many times the fee.
Hotels, and airlines, have promotional sales of bulk points which may be a better value than the amount you'd pay in fees to earn less miles.