I split my time between both UA and AA, and while both airlines are pretty equal when it comes to the poor experience with delays and cancellations, to my knowledge at least American isn't basing mileage earned on their FF program based on whether or not you hold their credit card (yet).

So for me, that is an upside to AA. I can only achieve top tier status on one of the two airlines, or maintain tier 3 on both, in my case all it does is push me to Ex Plat on AA and then once I have that rely on UA for backup travel.