OK
SATA3 I can fully understand, it isn't working properly yet and there does seem to be issues with the Marvell Chips that needs to be resolved.
But USB3 uses a different Chipset (on my AMD board it is an NEC one) so, unless there is a chipset limitation that we are not aware of, I have to agree with the original question, why only 2 USB3 ports when they are backwards compatible?