Photography is art - whatever method helps YOU take better pictures is the best method for YOU.
You can even take sports photos with manual focus (proof: there were sports photos before there was auto-focus) - it's just more difficult and takes preparation and planning (and missed shots).
The auto-focus on modern DSLRs is very fast and nearly perfect in good light, it's also very predictable when it will fail (low light, macro, etc.) - so there is no practical reason not to use it, but, manual focus does force you to slow down and think about focus. If this helps you get better pictures - or you just enjoy it more - then go ahead and take all your pictures with manual focus.