You don't need an SSD to get benefit from the transfer speeds of USB3. Decent hard drives support linear read speeds of ~160MB/sec. USB2 will throttle that to 40MB/sec after protocol overheads.
I've also run into this using external ethernet adapters, which I still want when I'm copying large files or streaming video via airplay or steam. My last 3 laptops were missing inbuilt ethernet ports. USB 2, while much faster than the wifi speeds available at the time, only supports half the bandwidth of gigabit ethernet.
External SSDs are probably the only time it matters, or even multiple drives connected through a USB hub.