BBMs biggest advantage and why you won't see it natively on tablets. (speculation)
BBM is all about communication. That may seem obvious.
But I think its superior to other platforms because its roots are in effective and efficient communication. Theres a reason they bring up the stat about how quickly BBM users respond to the message.
I think BB won't bring BBM to tablets because it bastardizes the quality of BBM as a communication service. Tablets will sit on the coffee table at home while your phone is much more likely to be on you.
Its the same reason BB has it so you can't opt-out of the delievered/read notifications. BBM is about communication and an active userbase is key.
Add to this the efficiency of BBs network (although its been put into question wether BBM uses the network on their new devices, although I haven't seen any info to back that up).
So I don't think it would be a good idea to put BBM on tablets. Unless its how they implemented it with the bridge. Thats genius and the way it should be. One device controlling your communication. I personally dont want my commincations going around to all the devices I have, its better for security and privacy and for more efficient communication. Also when I contact someone I prefer to know they got the message and its not just sitting unnoticed on their coffee table.