Those are WD's latest drives. With the cache they can do bursts up to roughly 200mbps in the right circumstances, but any extended transfer almost immediately drops down to 70-80mbps.
My guess is at this point its mostly a marketing tool, though with Sandy Bridge at our doorstep SATA III and USB3 are now becoming the standard interfaces like USB1 yielded to USB2.. and to this day there are plenty of USB1 (transfer speed capable)
devices still on the market. Some types of devices are limited by their design more than their interface, and at the moment mechanical hard drives are one of those devices.
Though... and it seems like there's always an exception..
I was recently reading a review about an interesting device that 'holds' a 3.5 inch mechanical hard drive AND a 2.5 inch SSD, and then fits in a standard 3.5 inch cage. The idea is to use the SSD as a sort of extended cache. The controller isn't a smart real time controller like on the Seagate's interesting hybrid drives, but rather it takes stock of the most used files on each reboot which seems to work with a larger SSD vs. the 4gb small SSD cache of the hybrids. Something like this, if paired with a SATA III speed SSD (I know of only one, the Crucial C300 I linked above)
could benefit from a SATA III interface.
Anyway.. my point is hybrid devices with fast burst speeds might eventually saturate SATA II but we're a ways off from that.