If it puts an uneven load on the HVAC system causing problems keeping other peoples' cabins cool it is, in the least, a bit inconsiderate to leave the door open.
If indeed it plays any role in fire safety as Bruce pointed out, going against the rules and leaving the door open is dumb and puts other passenger at risk.
Not 'griping', just calling it like I see it. Rules are usually there for a reason, not just to randomly inconvenience everyone.