I also think, that USB 3.0 is faster, I do not see any other reason 
It is not faster for mining, there is not all that much data being sent forth and back to the miner. Having a higher data bandwidth does not make the ASIC mine any faster.
It is true the speed of miner will not make a difference usb 2 is fast enough without issue. Only difference on paper is usb 3 has more power aviable so could allow more power for a stick.
But with all the mod's people show on board that is not always true. We have seen many usb 2 hubs people mod to add more power.
Personally i use a USB y splitter to get enough power. My USB 2.0 hub provide nearly as much power per port before sagging as my USB 3.0 port. (This experience for others will probably wildly vary depending on what hub they get) So if you really want more power to max overclock, you're most likely going to have to use two USB port if you want to push say a SideHack stick to the max, either way.
Otherwise if you run any normal stick, or a Sidehack stick at "regular" speed, i don't think any stick is going to max a single USB 2.0 port, even a regular end onboard one.
