Idk I think for the players who need crests and are pushing they can kill 2 birds with one stone. Getting 20 crests per run compared to 12 from a +8 or even 16 from a +10 is pretty big on top of getting that vault credit.
10s right now are filled with people who have no idea what they are doing. Literally 30 minutes ago I was in a failed +10GB where we had a rogue and a hunter not use Soothe a single time and landed under 5 interrupts by the time we got to the 2nd boss. They hilariously tried to gas light me for their failure.
I think 12s giving more crests and vault will let us hang around the higher tiers to avoid the bads.