I have carried an inflatable lower leg splint, but only as part of my SAR kit. I have used it multiple times and once on myself (leader fall). It is quick and easy to apply, but there are drawbacks - it can be punctured and if a change in elevation is involved in evacuation (think helicopters), you must adjust for varying air pressures. I think it too specialized to carry on a regular basis. One can craft just as good a splint from available materials - it will just take a bit longer.

Quite a few ankle "crunches" occur on trails. I am not sure if the incidence of ankle injuries is actually any higher when off trail. What matters is proper foot gear and strong lower legs, as well as attention to footing, which is likely to be heightened when off trail.