I use auto ISO for sports in the same way as MT Stringer. As there are only three inputs to exposure - aperture, shutter speed, and ISO - by shooting in M and limiting two of them (e.g., shooting sports wide open for separation and fast shutter for stop action) you are limiting the camera's decision only to the lowest priority parameter. Now the only thing you have to decide is does your eye evaluate light better than the camera? Generally, I think yes, but when shooting RAW, there is enough leeway to occasionally let the camera decide. And therefore to me, using ISO makes sense only when 1) you don't want to or cannot dial in your own adjustments, and 2) you have a need to fix both speed and aperture. If I were shooting Av or Tv I would probably not use auto ISO as I always want to fix at least two of the three parameters.
