Having an office doesn't mean you don't talk to coworkers. It means you get to control when you talk to coworkers. You can easily have 1-1 meetings in your office, talk about personal things and actually really hash out problems without worrying about annoying or distracting the poor sap 2 seats down. All office areas have shared space as well - thats why people meet up around the water cooler. If you want to spend no time socializing in an open office, its better to have your own office. If you want to spend lots of time socializing, its better to have an office since you won't be annoying the people who don't want to or don't have the time to socialize.
Many of the older Oxford and Cambridge colleges have two doors in one doorway: the external one for the 'interruptible/not' signal, and the internal one to keep the heat in when the outer is open. We have a term for it: 'sporting one's oak'.
Except for when someone doesn't use that model. I work with people who keep their door closed all the time, but they're not in a "don't bother me" mood.
Then they suffer the concequences of people not going up to them. That, or they want people "to know" that the door closed means they can come in - those who they haven't told will shy away. Its kinda a way of filtering, or they're socially inept.
