Well, what would have happened if it came open in flight.
I would not know... I guess that the pressure gradient over the wing would keep a door at the underside of the wing pressed closed.
When the door would open during take-off, it could get ripped off the plane, causing minor damage to the plane, increasing fuel consumption due to worse aerodynamics, etc. The door would likely end on the runway and need to be cleaned up to avoid accidents with planes using the runway later.
For planes with rear-mounted engines lost wing parts could get ingested by an engine, causing loss of the engine.