Neil's done a nice job of providing an example of why having more than one sensor matters.
I have a similar thought process. Ideally I would like to be able to anticipate action and set up with a composition that works. I can do that with one sensor, but it requires compromises and more missed shots.
We have a lot of Nikonian posts that are looking for an economical body and maximum reach. The Sigma 50-500 and 150-500 put you at f/6.3 in normal use. A Nikon f/4 lens with a 1.7 or 2.0 teleconverter puts you beyond f/5.6. These common lens combinations have only one working AF sensor. You'd hesitate to buy a setup with just one working AF sensor, and its even worse if you don't know that's the case. At least knowing you have one working sensor, you can work around the issues as needed.
The other thing to keep in mind is if you have only one working sensor on the D7100, there is a big opportunity for another camera that provides better capability at f/8 - and that suggests a future DX camera like a D400.