I somewhat disagree. I've never had a horse with a chip or bruise in a flat, even, unrocky pasture. If you just throw a horse out there, and there are things like holes, there is a very real possibility of a horse breaking their leg.
I've had a horse bow both front tendons in a flat, 'safe' pasture. I've had chips in feet from our pasture - Hilly, but not many rocks. Horses are pretty darn smart - They can sense a hole in long grass where we would probably walk right into it.