"Needing protection" is a pretty subjective thing- you're generally stuck with what the AHJ says.
I can think of situations where unprotected, open (insulated) condustors have been in place for decades without harm; I also know of places where schedule 80 steel pipe has prooven inadequate.
The photo forums have countless pics of otherwise code-compliant installations where the components were seriously damaged. Such damage is proof that the original install was not adequate- and, with luck, the repair will provide better protection!
Since you mentioned SER, I can only speculate it is being used to supply power to a service. In that case, the local PoCo rules will also apply. These tend to be much more specific than the NEC, and vary widely from place to place.
