In the past, when I upgraded bodies -- Rebel XT -> 40D -> 7D -- I always sold my old body. However, when I purchased a 5D3 three years ago, I decided to hang onto my 7D to use as a second body, dedicated to wildlife. When we travel, I shoot both landscapes and wildlife, and would spend a lot of time changing lenses. However, with two bodies I was able to leave my 100-400L mounted on the 7D for wildlife and use the 5D3 with a couple of lenses for landscapes, greatly reducing my lens swapping and also ensuring I was always ready for any wildlife that would be encountered -- grizzly bears and wolves will not wait for you to change lenses. 
While the 7D did not get used as much as the 5D3, I found the combination useful enough that I recently upgraded the 7D to a 7D2 (and sold the 7D) in anticipation of another visit to Yellowstone National Park this summer -- it will be my 6th visit to the park. I am really liking the 5D3 / 7D2 combo, as the two bodies are almost identical in control layout and feel. Both bodies will also auto focus at f/8, allowing me to use my 100-400L II with a 1.4x TC for additional reach, when necessary.


