While everything said is valid, the most important factor is the pax who is trying to get on. If he knows he can get away with a 20 minute check-in then 20 minutes will become the norm, this then become 15 etc etc.
At small airports it is not uncommon for passengers to turn up when the doors are closed and the aircraft has left or is leaving the stand and demand that the aircraft comes back for them.
I find it amazing that the less a so called business passenger pays for his ticket the more he expects and the more abusive he is likely to be when he is refused. While I refer to he, women are just as bad if not worse when it comes to being abusive.