I'm not convinced. I'm in an engineering area but if it's open plan then I'd still be able to see people, hear their (arguably more interesting than sales) conversations, and constantly have to split attention between monitoring the area around me and focusing on whatever problem I'm solving.

An engineer coming into my cube to chat a few minutes ago is what distracted me and now I'm frustrated and browsing HN. It's bad enough in high-walled cubicles; I don't know that I could maintain deep focus for meaningful amounts of time in an open plan.

I put a door on my cubicle recently and being able to mentally expand to fill my space while free from worrying about anybody else has done wonders for my productivity.