USB 2.0 gives you 60MB/sec throughput, though in practice, 30-40MB/sec is more common. An 8x10 photo, scanned at 720dpi (which is overkill) into a 16-bit TIFF produces a roughly 240MB file, and will take 8 seconds tops to transfer. The scan itself will take a whole lot longer than that.
In short, scanners don't come even close to saturating USB 2.0 transfer speeds, so there is no reason whatsoever to make USB 3.0 ones.
i7-3770K, Z77, 16GB, GTX 580, Win7