BBM is not a proprietary version of sms, its different. SMS is limited to 140 characters while BBM is not, you can send as much text as you want, including pictures, voice notes, files, gps locations (including map), contacts and appointments. BBM uses your data plan so you wont be charged extra to send/receive sms or mms messages (sms/mms messages charges when compared to data are ridiculous), and yes I know there are plans for sms.
The most important reason BBM is better than sms is that you can see the delivery and read of the message in real time. No D for delivered beside the message - it's waiting for delivery because the phone is off or out of the service area, and when the D shows you know the chime has gone off to say a bbm has arrived. When the recipient opens the message you get an R. No guessing like sms messages.
I can turn my phone off during a meeting, and the moment I turn it back on, instantly all the pending bbm messages arrive, not when the sms system gets around to sending it.