The economics of spoke protectors is simply - they cost around $3 online in singles and shipping isn't too bad if you buy it with other parts. They're even cheaper in bulk. And they can be free at some shops that might have a massive collection of these in a bin somewhere, or if you have access to old rear wheels. (You will need a tool to remove the freewheels/cassettes to gain access). Note that spoke protectors vary depending on whether it's a freewheel hub or a freehub. And among freehubs, the design can vary based on number of spokes. But installing one can save you a bundle of downtime and repair costs that are easily an order magnitude greater than the cost of the spoke protector. I think it makes sound insurance for folks who might not have a rear derailleur always in good adjustment.