To use BBM you need to use the BlackBerry Bridge which provides a Bluetooth connection between the Playbook and your BlackBerry smartphone. Since the clearance sale of Playbooks nationwide there may be some consumers who don't necessarily have a smartphone to bridge with and only have the Playbook they got on sale or as a door prize at their company Christmas party.
Also consider, a tablet is a device that is sometimes used by multiple people in a household and a handheld device is much more personal. I wouldn't want my brother reading my BBM messages on the household Playbook. The Bridge is more of a novelty. How often is your BlackBerry not close at hand even when at home? Would BBM be as engaging of an application anyway if it was only used on a WiFi PlayBook? I don't think
BBM would be the same experience without my handheld.
I can see groups being a good application to have on the PlayBook - BBM Bridge still doesn't support it, either way this would be the next feature that should definitely be built for the next release of BBM on the PlayBook.