Like all other laydown transmission MM cars, the body serves as a protectant to debris. With the slipper adjuster needing to protrude through the body, a gear cover is kind of unnecessary and overkill.
Today most tracks consist of finer substrates and they try to ensure no "rocks" reside on them. The most you'll likely ever get in the spur is larger grins of sand/dirt, but not much of anything else. And if you did get something stuck int he spur and damage or break it, they'r like $4 and most people usually have a stockpile of them so it's a quick and easy change to make.
I think you're overthinking this a bit.