Though these things are pricey for little plastic bits, the good news is that they should prove pretty durable, unlike the silicone ones that are cheap but usually split or wear out quickly. If you can catch a sale, it's not terribly painful. I was able to fit these on my SRAM carbon cranks with some slight modifications.
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.