I ride a lot of technical terrain with lots of big rocks and step ups. I'm always beating my cranks into the rocks. I got a new 29er a few months back and put a set of these on my Sram XO crank. They fit perfectly and no dings in my crank!
I bought these to protect my new Shimano CX-70 cross crankset. The boot fits tight on the drive side but is slightly loose on the other side. I like that they keep my cranks from making direct contact with rocks and such as I often ride my cross bike on MTB trails. Why not? You're going to buy a bunch of stuff, for [$] or so, you may as well add them to your shopping cart. You won't regret the purchase.