Not sure of the specific mechanism, aircraft types and pre-flight checklists you use, but my airline has a specific preflight check item for BOTH cockpit crew and the purser to verify the flight deck door latching mechanism on ground, as well as to test the external override mechanism. If the door is not fully latched and latch engaged, there are two additional visual indicators that point this out to the crew (one each located inside and outside the cockpit) and verification of the external indicator is one of the pursers checks prior to notifying cabin secure.

It is not as much as security issue than a contamination issue. In addition to being a security barrier, the flight deck door also serves as a fire door and most importantly as a smoke barrier. The cockpit runs on a separate ventilation system than the cabin so leaving the cockpit door open would permit cross contamination in case of fire. Furthermore, in the event of decompression, the cockpit door serves as an important barrier towards foreign objects entering the cockpit and distracting the pilots.