I absolutely agree. Team rooms seem to be the optimal. The room gets to set the rules. It's amazing how different team cultures grow, develop and optimize for the team composition and the work they're doing.
Provide a few quiet ultra-concentration nodes that people can reserve and use and conference rooms for inter-team coordinating and you're set.
You get the benefits of quick communication and collaboration, without the downsides of other people's/group's conversations disrupting what you're working on AND the ability to comfortably complain that your teammate is being disruptive...which is socially harder to do with strangers/people in other parts of the company.
If your rule is "put headphones on if you want privacy" you're doing it wrong. If you need to have "flow time" in the afternoon because the warehouse you've stuffed your people into is too noisy to get any real work done (so you have to schedule time to actually concentrate) you're doing it wrong.
Some places even go so far as having re-configurable team rooms, they can change size and shape to accommodate growing and shrinking teams without too much fuss.
Efficient for what or who, I wonder?
What I generally see is I find out more about my co-workers lives and their interests than the increase in productivity and innovation from the team.
I fully accept that there are confounding variables. But, I can't help but also accept that open plans are not the doomsday device that they are often painted as. They are yet another of the many variables that go into how a company is running.
Someone pointed out you do get to know your coworkers better, but I'd argue you just get to know them better, faster.
Cornell did a study that showed that people interact less when they have offices. Their definition of "frequent interaction" dropped from several times a day ad hoc (with open plan) to several times a week in a meeting. That's some data supporting that it does in fact improve collaboration.
Archive.org is down right now but I can supply a link to the study later if you want.
We designed a ton of cubbies (like in your university library), 1 person private rooms, 2 person conference rooms, etc. in our office to accommodate for the fact that many people need to more privacy and quiet than just headphones. We also break up the main open plans to help quiet the noise and distraction.
We have about 45,000 sq. ft today, and will be adding another 45,000 sq. ft this year. When we do, there will be much less open floor plan. I do think there's a happy medium, with team rooms of 6 to 15 people, depending on role and requirements.