I have not direct experience, which is why I prefaced my statement with "I have always been told." Interesting. I don't agree with the helmet analogy... If the helmet cracks, it cracks. Try wearing a steel helmet that will compress to a new shape on impact. Not only does it dent your skull into a new shape, but it becomes stuck on your head. I would also guess some "steel toe" boots are made better than others.
Thanks for the link. General criticism of "Mythbusters" aside, I think there is some information in that article to support problems with wearing steel toe boots around a horse. They were going for "amputation." I am just talking about additional problems from the steel being bent.
"400lbs from 3 ft: more deformation in the steel plate, but only damage to frangible foot was a broken metatarsal (big toe). Adam: “I want to see some toes cut off or crushed beyond all recognition” * 400lbs from 6 ft: a lot of pancaking of steel cap and lots of broken bones beneath, but no toe amputation."
Of course, enough force to bend the steel would do plenty of damage to the foot anyway. However, if 400lbs @ 3ft can cause the steel to break a toe, the steel would seem to be bent into the toe itself. That would make me wonder as to how you get the boot off? Wouldn't the toe be trapped by the steel toe?
Again, these guys were looking to disprove an amputation theory. They didn't speak to additional damage potential from the steel. It would also be difficult to relate their tests to the force a 1200lb horse would exert when stepping, walking or trotting across a toe.
I don't have any experience one way or the other. Never heard of anyone being stomped on while wearing steel toe boots. (Although I use to wear a pair of steel toes long ago and did have some dents and deformations in the steel). There are plenty old wive's tales and I have no doubt this is one of them too. At the same time, I personally believe a horse is capable of bending that small piece of metal and wouldn't take the chance. I have seen them do more damage to bigger things.