If you aren't concerned about extra award space, United Club access, or other perks, you might want to consider earning Chase Ultimate Rewards, which can be transferred to United almost instantly. (upgradedpoints.com)
Bottom Line: Access to extra award space on United might be the best perk offered by a co-branded airline card. (upgradedpoints.com)
In this case, UA was giving extra award space, but only to their card members. (travelisfree.com)