I'm way more likely to ask a co-worker over to my desk to show them something or have a discussion if they're a few feet away within sight and earshot. In an open office I also overhear a lot of pertinent information -- I'm aware of issues that might be relevant to me as they develop, I don't have to wait hours or days for someone to communicate them in an email, issue tracker, or meeting.
There's a cost in terms of readily available heads-down concentration time to be sure, but if I want to mess around with math formulas or something I pop on the headphones, go to a quiet room, or do it at home. It's much harder to replicate the benefits of an open office environment in a closed one than the reverse.
Why does it make sense to optimize for the former instead of the latter? The benefits must be studied in context of how overall productivity and bottomline is affected, not just easy it is to start talking to someone.