For standard HDDs, 6Gbps is redundant. At max transfer rates, you still won't even saturate 3Gbps. The only thing you need 6Gbps for is if you've got an SSD installed.
That said, the drive you got is *compatible* with the Sata3 standard. It just will never ever actually transfer that fast (due to limitations of mechanical drives).
On previous gen "normal" Sata2 drives, the 3-1.5 switch was definitely one of the features. Now that you mention it, I'm not sure if it'll still be the case with newer Sata3 drives. I'll do some research and post up.
