I think the folks who say it isn't a black or white issue are probably right.

If you have the opportunity to keep your horses like this:
http://www.nakedhorsemanship.com/dancing%20barefoot.htm (video of horses in a paddock where it is all hills/rocks and they move a lot during the day)

...then their feet will likely toughen up to deal with any environment you can throw at them (just like callouses on our hands if we use them for hard work - and I'm pretty sure humans are not selectively bred for calloused hands either, just like horses are not selectively bred for hard hooves). However, most people don't have the opportunity to keep their horses in this type of environment. Thus their feet don't toughen up, thus the need for shoes when on rocky trails or whatever.

"Common knowledge is the enemy of scientific advancement." - J. Gore