Yes, a 6Gbps interface is about as pointless as a USB3 floppy drive. Even SSDs will have a hard time saturating SATA3 currently. For platter-based storage, probably the only effective means of using such a link would be for hybrid disks. Many high-performance storage systems have tiered storage: RAM-based --> SSDs --> HDDs (---> Tape). As data ages and is accessed less frequently, it's moved to a slower (but cheaper) storage location. A hybrid 3.5" drive that has 1GB of RAM, 32GB of flash AND 2TB of platter storage would be something that could be very nice to stick on SATA3. Perhaps using some semi-intelligent data location algorithm to stick (a interm copy of) your database/dlls/etc high-frequency stuff in the RAM for super-fast access, quick-to-access info in the flash (such as boot-required windows stuff, perhaps your favorite game), then store your MP3s, DVD rips, and your photo gallery, etc on the platters....
Of course, anyone that has the money and a bit of know-how has probably already set up a RAM Drive, an SSD, and mass storage in their computer anyway. It would just be convenient to offload all that hassle and headache into a "black box" of a hard drive.
Mine's with the patent application in the pocket, so I can troll on it later.