If you have a platinum and you open a cobalt account you will then have to MR accounts.
Cobalt: MR Select Tier
Platinum: MR First
The reason I got the Cobalt is the ability to transfer points to BA and air canada at $1:3 Avios or Aeroplan. With the platinum it was $1:1.25. So it is a significant improvement.
So my current spending plan:
For merchants that take AmEx inside of Canada:
Food & Drinks: Cobalt
Everything Else: Platinum
And for Amez SPG it is $1:2 avios or aeroplan, so even better than Amex Plat for non-food and drink merchants.
By the way who want to convert SPG into Avios or Aeroplan...