When I open/close a door in real life, it usually makes a sound.

When I open/close doors in other games, they usually make a sound.

It's another old relic that has been around since OFP and was probably classified as "it has always been this way mkay?" ;) .

The missing sounds have quite a drawback in MP as you gain no audible confirmation of e.g. an enemy entering your building. It's also less atmospheric.

Sounds can be implemented via scripts executed when the animation triggers since OFP.

See this addon by Mondkalb for an implementation of (mp-synchronized) door sounds:
http://www.armaholic.com/page.php?id=19917