There is a different between points, miles, and cash back. Citi has AAvantage. Unlike other travel credit cards you get miles instead of points. Points are used 100:$1, miles on the other hand can be 12,500 miles for a $750 dollar flight. Thats 83:$1. Meaning each $200 dollar phone bill could potentially be worth $12 in return. Therefore, you make an additional 2 dollars over what AT&T is giving you as a 'discount'.
Edit: Before anyone states 12,500 miles for a $750 flight is not possible, it is. I flew first class from Los Angeles to Orlando for $12,500 miles and that one-way flight was like $753.xx at the time of booking.
So for this reason, my family is not enrolling in AT&T's 'deal' on our Unlimited Plus plan. Its not worth it to us. The extra 10 miles a month adds up over time.