Which airline(s) you have closest access to is probably a major factor here.
I am between hubs for the three legacy carriers. Therefore, I see Amex Plat as a companion with Delta and Citi Prestige as a companion with AA. It may complicate things, but I gain flexibility in finding the most value from every flight I take. Based on my location and some other factors, I find the two cards complement one another so well, that I want both.