It lowers the overall productivity, because the people who most need to be left alone are the people who do most of the work. Worse, it's a long-term detriment to each developer's individual capabilities, as it trains them not to use google or read documentation.

You can still interrupt someone without an open plan office, there's just a slightly higher bar that prevents most people from asking the knuckleheaded questions, and keeps constant non-conversational visual and auditory distractions to a minimum.