How about the use case for upgrading to new drives?
Let's say i have 10x8tb drives and 2 8tb parity drives. Your case is quickly running out of physical space.
Right now if I wanted to introduce 14TB drives for example into the array, the solution is usually to push them into the array or parity disks until you get enough to be able to handle the full array size to copy. In the mean time, you are not utilizing the disk capacity above 8TB for these drives unless I am missing something.
If you allowed a second array, you could spin this up with 2, 14TB drives and use it on day one... Adding a Third for parity and get running pretty quick with no downtime on your first array still utilizing disks. After time, you could either run the arrays in parallel or start copying data to the new array as you can to eventually decommission the first array.
