There is no benefit in connecting a USB 1.0 or 2.0 device to a USB 3.0 host.
USB 3.0 (if nothing leapfrogs it) will probably knock Firewire out, but, as yet, there is nothing to plug into it that won't work just as well in a USB 2.0 port.
That is for the future (at least I hope it is)... until those devices are available, there is no point in buying expensive 3.0 adapters.
There doesn't seem to be any rush, in either the domestic or pro market (but see, and probably watch, RME) to produce 3.0 audio devices. This may be because they see no benefit in doing so, and there may, in fact, for 2ch audio be no benefit. But soon all the PCs will be coming with blue USB ports, and then people will be wondering why their PC is 3.0, but their DAC is "only" 2.0, so it will become a marketing issue.
EDIT: I've been telling myself, "surely someone must have made a USB 3.0 DAC." Haven't found one. Anybody?