Frankly, whether people will admit it or not, most of the time you end up in an environment with a private office for status reasons, not business reasons, and status is not a particularly compelling argument for a specific office configuration. In fact, status has the downside of causing people in the company to work toward status instead of working toward results.
Many people call out the fact that in open plan environments concentration can be hard, and I recognise that to an extent. However I think once you get used to working in such environments concentration is perfectly feasible, and the benefits of ad-hoc communication far outweigh the negatives.
I also find the ‘status symbol’ factor of an office unnecessary and somewhat offensive. Seniority already earns good compensation, responsibility and challenge; adding ‘perks’ such as an office just creates a them-and-us barrier that belongs in the feudal system of the middle ages. However, even worse is when people are told they ‘have’ to take an office in order to advance their career (and yes I do know a guy that that happened to.)
The best addition I can make to this area from my own experience was the ThoughtWorks office in London. This was a total open-plan office (at least while I worked there), from the CEO down, with lots of meeting rooms of various sizes and also break-out (non enclosed) areas for informal discussion. Even the HR people (who probably most need an office by function since they are the ones having the most legitimately closed-door meetings) used the communal shared office pool. And when one wasn’t available, well that was a good chance for 2 people to go and get a coffee together, stretch their legs and talk privately on the way.