...you don't ever have to clip them if they play and run on concrete/cement a lot because they'll just file them down far enough...

That is true to an extent but it depends on the dog's conformation. Dogs with arched toes (not sure of the exact term for what I'm trying to say) to wear their nails down with lots of running, dogs without arched toes do not. Most Rottweilers for instance, can't wear their nails down because of the shape of the breeds' paws. I hope this makes sense, sometimes I'm awful at explaining things...lol.