I think the main factor is whether it's the same vehicle or whether the layover is a relay. The vehicle that pulls into the stop may not be the first vehicle leaving. Most bus and tram systems I'm familiar with the vehicle pulls in, disembarks passengers then goes somewhere else to lay over. The next vehicle out is another vehicle that comes out of layover into the stop to pick up. Also most termini I know have shelters and seats, so nobody is waiting in the weather if they don't want to.

What's happening in Melbourne is that because it's a double-ended tram system, most stops have only one stopping position from which the tram doesn't move between arriving and departing, nor does another tram appear, so they might as well leave the doors open because no other tram is going to be the next tram out.

Tony P