Some of the advantages BBM has over texting is no 160 character limit, group chats, ability to send pics/vids/voice messages without needed to send a MMS (believe it or not, not all carriers even today offer MMS messaging options, especially in the developing world). Blackberries seem to be very popular in the developing world as well. I sold my old Tour on ebay to a guy in Vietnam who unlocks them and resells them.
Whatsapp has the advantage of being able to msg people on android or ios so basically a universal txt msg app, BBM is hardly a compelling reason anymore.