Why obviously?
USB2 is more than capable of simultaneously handling 3-5 straight blu-ray rips. depending on the bit-rate of the movies.
apart from the initial load-on of the files, you won't see much gain by using USB3. and if you're downloading off the net, you won't see any difference.
granted the price premium for USB3 over 2 is rapidly disappearing and almost non-existent. so there's really no reason to not get 3.