My 2 cents: a lot of people have a bloodlust against private add-ons because they think they ruin the game or whatever. Sure. That doesn't mean that there is no use to having a restricted add-on, a primary use that I know many add-ons have used in the past is for beta testing. It's simply easier to upload an add-on to RTB as private and send your testers an ingame link to RTB-XXX than it is to have them download it from dropbox and have it unmanaged by RTB. Due to the volatile nature of beta add-ons, it's easier to allow people to update them using an ingame interface than mentally keep track of who has what version and tell people to update.
It has a value, but implementing it properly also opens up a new can of worms of issues (like whether private add-ons should be reviewed or not, and what kind of privacy should be expected for them). It might be done, but it shouldn't be a focus right now.
Also, a second note, I would say that private add-ons aren't counter-productive to the goals of BAM. Look at GitHub, their goals are open source code for all projects and the sharing of knowledge, yet even they support private repositories. It's an industry standard for version control systems (like BAM) to support private projects.
GitHub isn't really comparable, since their business model is to get you hooked on the free version for open-source projects so you'll convince your company to purchase the paid version. I don't think charging for service is something which is on the table right now (especially considering things like the Danish labour laws). :P