At twenty employees with several partners, a flat model works. Typically however, that approach only scales so far. As an organization starts hitting 30-40 people it starts to become hard to maintain informality and avoid territoriality. At 150 or so, you hit Dunbar's number.
If the company does start to become bigger, and groups in charge of certain aspects of the products start to form, is it possible to still apply this model on the smaller formed groups?
Then the weekly leader of each group meet and therefore encourages employees to maintain the "bigger picture" of the product.
It's also a reason to make sure your middle managers are not paper-pushing bozos.
The idea that a middle manager has to be an entity that generates meetings, paperwork, stupid rules and hot air is prevalent, but by no means necessary. Hard to believe, but trust me - the good ones see their role as exactly the reverse.
http://en.wikipedia.org/wiki/Dunbars_number
Don't get me wrong, it sounds like a great place to work and an admirable way of managing people.