Open office plans are wonderful for the micromanager. Everyone's screen is for all to see - and the moment someone happens not to be looking at an IDE, it's an opportunity to ding someone on their performance review.
While I personally am hyper-focused at work (perhaps too much to the point of tunnel vision), I know peers who have received negative comments despite delivering stellar work - all because they look at HN or doing online shopping from time to time.
It's very easy for managers to keep track of their herd, see which face they don't like and flex to the colleague/competition using the always important "how many workers I have under my wing/foot" metric.
I think the only downside of open offices is that productivity takes a significant hit and makes worker health and morale a nightmare. But other than that they are fine.