You have to look at this issue from a marketing and a standardization perspective.
While it's true that mechanical HDDs will never saturate 6GBs, the fact remains that this is becoming the new standard. So, in the same way that the SATA 1 spec drives have all but disappeared, the SATA 2 mech drives fall into the, "this too shall pass", category.
With that said, the manufacturers will likely update the HDD electronics to the SATA 3 spec, but probably won't be able to do very much to improve the mechanical transfer speed. So then, you just won't find SATA 2 (300Gbs) HDD as the new spec takes hold. But, it'll boil down to this, "same s***, different chip".
Hey they're all backwards compatible, no big deal.
And the answer you're looking for is this; buy a board with the SATA 3 spec if you like, it will come at a slight premium. And, depending on how often you buy a computer, the next one probably won't give you a choice, it'll be SATA 3.
If you really have no intention of ever using SSDs, then you should buy a board with the features you do need and want, at a price you're able to pay. The kid down the street will always be running his mouth about how his machine is better than yours, no matter what spec you have on the SATA buss.