I am building a 1 x 10 out of a Trail SS and on my other bikes the shifter cable is covered on the bottom. I was thinking of running housing over it and wondering if that is necessary and if anyone is having problems with the exposed cable? I imagine a rock popping up would damage it fairly easy.
The cable wont be damaged by rocks unless you grind on it a long distance... but that would probably also grind through a housing... and if that much damage was to happen, your cable wont be your main concern.
Full housing can help prevent dirt working it's way inside the rear housing loop though, making the need to replace the cable coming later into the bike's life. But that depends of the conditions you ride in.
like other people said, if anything got in there, you'd have scratched frame, crank damage maybe chain damage or derailleur damage. Of all those things the cable and cable housing is easiest to replace.