What, if any, equipment on a commercial airliner is (allowed to be) inoperative most of the time?
If a system goes south while sitting at the gate or the ramp, that brings on a cockpit indication, the crew will break out the MEL/CDL using whatever protocol or procedure they follow and if it's not covered, they are grounded until the system is made serviceable again and will take a significant dispatch delay, or worse, a cancellation.  

The OEM will try to get as many items covered under the MEL as possible and will continuously try to expand it, because it has a big effect on overall dispatch reliability.  An unreliable system system or subsystem that has a lot of failures will just be maintenance cost issue (and possibly minor delays) if it has MEL coverage, but if it doesn't, it becomes a cost plus dispatch delay/cancellation problem, much more expensive.