Originally Posted by
storckm
That was the reason for the disks in the 1980s, and it's the reason why they still make sense, at least for bicycles that might belong to someone who doesn't know what a limit screw is, let alone how to adjust one.
Just out of curiosity, how often do you check derailer alignment and limit screw adjustment? After every rocky or muddy ride? After every time you come out of a store or coffee shop, in case someone knocked your bike down and then leaned it back up against the wall?
There's a ton of ways a derailer can get mis-aligned, and spoke protectors (aka dork discs) provide protection against all of them.