If the MEL states No Dispatch then that should be it.
However, MELs are often not that simple and sometimes require a certain degree of interpretation. The capt is always the final authority and in the light of all factors considered he may deem unsafe a condition where the MEL would permit release. But he cannot depart where the MEL says no.
In this case it depends also on what is considered inop. Is it the lock itself, the release button, or wouldn't the door close at all. The major issue here is not the prevention of a terrorist attack (this is a rather silly debate) but the isolation of the flight deck in case of smoke or fumes in the cabin.
If you have the slightest doubt always fall on the conservative side.