These Goflex drives don't use adaptor cables, they use adaptor bases, with a variety of connectivity options, depending on the base. Given that a FireWire 800/USB 2 base is $50, I expect the Thunderbolt version to be even more expensive.
The whole idea of the GoFlex line of drives is that you can move the drive from one base to another - or move the base from one drive to another. How useful this is depends on your workflow. Surprisingly, the FireWire 800 base does not include the two FireWire 800 ports that make it possible to daisy-chain FW 800 drives - a common feature on most full size FW 800 drives. This means that if you do have a chain of drives, the GoFlex would have to be the last drive on the chain. Which makes me wonder if the Thunderbolt version, when it arrives, will have the same limitation.
While I find the concept interesting, given the added expense of these GoFlex bases, I'm hard pressed to see what advantage they offer over the usual multi-interface drives.