To be perfectly honest, I was along the same train of thought as you were in regards to the lack of Band 12 on BlackBerry 10 devices until I realized it was only primarily T-Mobile looking to utilize it in the very near future.
My guess is that if T-Mobile and BlackBerry rekindle their business relationship and you start to see the Big 3 in Canada utilizing Band 12 within the next 2 years (rather than the 5 as per the licensing agreement), you will see BlackBerry roll out a device with focus on using Band 12 (amongst other bands, of course).
I cannot help but wonder if the new BlackBerry slider will include Band 12 LTE capability or not. My bet is on no, but then again, we all could be surprised.