Is there any benefit to plugging a USB 2.0 device into a USB 3.0 port?

Actually, yes, it will be faster by a small margin. You will only see gain if the device in question can dish out a higher bandwidth over another interface like ExpressCard or PCIe. for instance a modern 7200 hard drive in a external enclosure could more than saturate the USB 2.0 port. If the enclosure is a USB 2.0 device, it will be operate with more of its bandwidth when plugged into a USB 3.0 hub, but not nearly as much as if it was a USB 3.0 to USB 3.0 device to hub link (with a USB 3.0 cable).

At least on my laptop, USB 2.0 external 500 GB on USB 2.0 gives me about 19–23 MB/s and up to 25–32 MB/s when connected to a USB 3.0 express card. So I am getting both a higher minimum speed and ceiling when the same USB 2.0 device is on the USB 3.0 hub. I think the controller is probably more efficient on USB 3.0. When I plugin a USB 3.0 thumb drive on the same ExpressCard USB 3.0 hub though, I get up to 122 MB/s.

So short answer; yes, a small increase, but not nearly as fast as native USB 3.0 links.

Since your USB is optimized for USB 2.0, using a 3.0 will see no improvement because it simply cannot operate at 3.0 speeds.

USB 2.0 has a maximum speed of 60 MB/s USB 3.0 has a maximum speed of 625MB/sec

From Wikipedia's Article on the Universal Serial Bus:

Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet.[62] This is 70% of the total bandwidth available.

Based on this, you can see that USB 2.0 devices just are not capable of the speeds 3.0 has to offer.

TL;DR version: You will see no benefits

One advantage could be that USB 3.0 can supply more power than USB 2.0.

I have some doubt whether an USB 2.0 device could use that power, as it would be designed for a USB 2.0 port. On the other hand many USB 2.0 devices exceed the specified power and get away with it (mostly external disks when they start up).