Interesting responses. I've struggled with this for the past several years and don't have any advice.
I can tell you that my decision varies by sport.
Volleyball -- shooting through the net to get the faces of the players -- almost always use single point, which does seem to help keep the focus on the player rather than the net. But, often have a problem with the autofocus picking the back of the players on the near side of the net, rather than the face of the players on the far said.
Outdoor sports -- baseball, soccer, track etc., I often used the expanded points and pray that in the day there is enough depth of field to get the subject reasonably in focus.
Basketball -- either single point or expanded, just depending on what seems to be working the best at the time.
Swimming -- same challenge as others, especially because the swimming pool I shoot is poorly lit so often shooting wide open f2.8 with a 70-200. Use single point mostly.
I think I have a better keeper rate with the 1DX II than the 7DII, but honestly I'm not totally convinced of that. The main reason I went to the 1DX II was the better high ISO performance inside. Outdoors, I actually find the wider spread and extra reach of the 7DII makes the difference between the two cameras about a tossup.
While using a longer lens can help some, in my experience it's not always the perfect solution. With a longer lens, you can get a sharper image when it's in focus, but following the action is a lot more difficult and when it's out of focus, it tends to be really out of focus and not usable.
I'm in a somewhat unusual position because I shoot sports for a small college. That means that in most games, I only care about getting one team in focus. For example, in soccer, if the area of sharpest focus happens to be on an opposing team player, the image is useless to me, whereas someone shooting for a newspaper or other media, may not care as much which side's team has the sharpest focus, as they may be more interested in the overall action.
Finally, a bit of a rant. Sites like DPReview take a lot of grief because they are consistently critical of Canon's autofocus system. Some people on this site say they just don't know how to use the Canon autofocus. Maybe, but then I guess I don't know either, because I have some of the same problems and clearly, the number of professional photographers using single points would indicate that a lot of people find Canon's autofocus less than perfect.
Someone needs to design an autofocus system that allows cameras to lock onto a subject using single point autofocus and then have that point follow the subject around the viewfinder.
