Mainly just to force the lens to stay wide open even if play moves from a shaded area into the bright sun.
But yeah you might be right. The autofocus on my lens seems to be reasonably quick, but that just might not be good enough for quick sudden movements in play. I'll have to practise and play around to see if it's a user problem or an equipment problem. In the latter case I may have to stop down. In which case manual with auto-ISO becomes even more important.