Are switch guards in the MEL?
 

I don't have experience with 121/135 ops or MEL's (yet) but I'm curious;

Are the plastic switch guards required or mentioned in MEL's? What happens if one or both of the joints break and the cover falls off?

No experience with this, but I believe if the guard is broke, the switch is considered inop. Which may or may not be a no-go.

It’s a guarded switch for a reason.
Guard inop switch inop.
If it’s a guarded switch and it has a little break wire securing it, break wire broken needs to be replaced.

Not all guarded switches are break-wired on all aircraft.
Otherwise, I agree.

Many airplanes have a significant number of guarded switches that are used on every flight, the guard just prevents accidental activation of a switch that could cause damage if activated at the wrong time.

Switches that are safety wired are used in abnormal situations.

Joe