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.
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.
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