SATA-III was developed because the very fastest SSDs were bottlenecked by SATA-II. But hard drives just don't spin fast enough to get anywhere near even SATA-II speeds, so they don't benefit from SATA-III.
The manufacturers who build the SATA interface chips are all switching over to SATA-III, so most of the new drives coming out use those chips whether they need it or not. It's the same situation as with USB - all mice and keyboards now use USB 2.0 even though USB 1.1 was more than fast enough for them. But nobody makes USB 1.1 chipsets any more, so even pokey slow devices are using the 60MByte/sec USB protocol. In fact they'll probably all end up using USB 3.0 at some point in the not-too-distant future.