Chris

Common procedure on modern aircraft is that the cabin door is locked when shut, but can be opened via a keypad. This allows the crew to come and go as required without needing to be let in all the time. But post 9/11 security changes have resulted in a manual lock being fitted on the inside of the door, so if there is a hijack in progress in the cabin, the door can be locked in such a way that none of the crew outside can be coerced into opening it.

Whether that's a good idea depends on which you think is more likely: a terrorist trying to force cabin crew to give them access to the cockpit, or one of the pilots going suicidal and locking everyone else out.