I guarantee there are a lot more photographic profesionals who think, "I need the added security of a backup flash card" than who think, "I need 12fps continuous shooting".
That's sorta the point I was making. If you need the 12fps continuous RAW shooting, pro-level weather sealing etc., there's only the one option.
Therefore, limiting the speed of the 5D Mark III with two cards in RAW mode would be a very effective way to encourage some of those folks to spend the extra money on the 1DX.
If they don't need all the other stuff the 1D bring to the table, but they need the security of the backup card, then the 5D is perfect for them. It's a selling point of the 5D versus the 6D.
I dunno, I doubt many 1Dx owners would be 5D3 owners instead had both cards been full speed, but maybe I'm wrong. Shrug.