But you could also use an tb-ssd. You can do an Fusion Drive with an intern hdd and tb-ssd.
If you are willing to rip your iMac open, the blade SSD is likely cheaper than a TB-SSD and the cable to connect it. Plus, the system wants to do this automatically after you install the second internal drive. Like I said, this is great news for people that want to add Fusion themselves. Also, unlike Apple's upgrade, you can use a much bigger internal blade SSD.