I think it would be neat, though not necessary for most of our setups, to allow for speed boosting when drives in the RAID-F array are not being used.
So a 10 disk array, 8DRU/2 PPU, that is less than 20% full can utilize disks 1-4 & 5-8 as Disk 1,1,1,1 and Disk 2,2,2,2 all data being written simultaneously, and read in stripes. (A 300% increase in read speed with a negligible difference to write speed.)
21-45% will utilize disk 1-2 3-4 5-6 7-8 as Disks 1,1 2,2 3,3 4,4. Depending on the strip size will dictate whether or not the speedboost is used on "smaller" files.
The reason I removed SSD's from this mix is because of the down time it takes to TRIM to reinitialize the blocks for writing.