20-40 employee bracket here! All of our employees are billable (sort of). It's hard to explain our structure but needless to say our CEO/COO/Operations Director/Consulting Director/CFO all work as a team and are never really limited to their "designated" positions. Our Operations Director has been billing on a project for 4 years.
Our company has an interesting dynamic. I would say everyone has the ability to "speak up" but we have designated management roles. I speculate there are some reasons for this:
1. Leadership - "junior" people need a certain level of mentorship. Many of them don't understand process and need managers to guide them on those processes. Hiring the quality of people (even at "the bottom") that your firms may attract is VERY difficult for the other 99% of companies like us out there. We simply don't have the whole techie seen watching our firm day to day and thus don't attract them to our company. Hiring SUCKS. Is that our issue? Sure. But this utopian business world isn't always possible -- even though we want it to be.
2. External client view - Our company works almost exclusively with blue chip companies. They expect to see CV's with years of experience and fancy titles like "Principal Consultant", "Management Consultant", etc. etc. With these titles comes the responsibilities of managing people. When I used to work for the largest biz software vendor in the world I remember there was a title "Engagement Architect". I was basically told that this position only existed because the client contact (mainly the CIO) needed a contact at our company that was on his same "level". Also, some people flat out do not have soft skills.
I think the overall point is this... this is not a lesson in how to run a better business. This is a lesson in how Jason is happiest in running his small business and wants to keep it a small business. Many of us love the agility of being in a small company and fear if we add more numbers we will lose that agility. (and that's probably true) 37signals has only increased it's employee base to 26 over 11 years. With the amount of press they have drawn there is no reason why this number shouldn't be 260. For the record, I'm not saying that it should be 260, I'm saying that they have probably favored higher profit margins on their existing employee base rather than to scale the business out vertically.
Fred Wilson once wrote "Marketing is for companies who have sucky products." In order to do this with 26 people, you need to find 26 very good people. Do you know extremely difficult that is to do? It's even difficult with just 2! Face it, most people SUCK at business. I applaud Jason (and others) who have had the fortune and ability to do such a thing.