When you open the BBM settings for Android, you’ll have the following option:
It means that BBM has to persistently run in the background in order for it to get going and receive new messages. This is in contrast to how other instant messaging apps like Viber, WhatsApp, and WeChat are operating, and is because BBM ignored to use the Google Cloud Messaging service.
As a result — clutter in the notification bar and notification drawer:
That, of course, is on top of the waste of RAM dedicated for the BBM app as it persistently runs in the background. Nice app you got there Blackberry!