I am not a proponent for hard separation either; I would prefer to have a sofa and a coffee machine and white board in my room and have colleagues stroll in when the door is open. It would not always be open, though.

That "appearing as not working while on the clock" is, to me, a foreign fear, and not something I've ever really experienced at the places I've worked in. I know that I (and the teams I've been part of) have been plenty productive without pressure to put in the hours. I think that a culture of putting in the hours would have been counterproductive, in fact. My work has not been flipping burgers, but designing and creating complex systems. It doesn't work out that well under time pressure.