Watch the installation video on their website & you can see how these are held on. No way are these going to protect your tub from rocks - in fact they will actually cause damage to your tub because they are bolted directly to it!
They are more along the line of a brush guard maybe. They do look like a nice option if you are only looking for a set of steps though.
If anyone is looking for an example of a rock guard/slider check out any of the JKX videos Eddie & Cindy made. Those demonstrate what you want your rails to protect you from.
Again, if you aren't going to need that kind of protection then just go with the bolt-ons. Just don't expect them to take any hard hits without damaging your tub.