This is an opportunity I have taken to share with you The PIN-based invitation model on BBM is something that makes it different from WhatsApp as users get an absolute authority on adding people to their BBM networks, unlike WhatsApp that lets anyone who has your phone number reach out to you. While the PIN-based invites may be considered an advantage, I see it as a bane. Keeping someone away from your BBM does not mean that you would be able to shut off all communication from that person. All a person needs is your phone number and then he has all the other ways – from calling to messaging to IM on WhatsApp – to communicate with you.
BBM is only one of the ways. A common belief is that a BBM PIN, at times, is shared when you don’t want to give your number but want to stay in touch and maintain communication with a set of people. In such cases I would recommend
Google Hangouts (Google Talk) instead, which I believe Android users would be quite used to. You can take their email address and add them to your Hangouts. Here you also have an authority to choose and reject. Another purported BBM advantage washed away.