From my understanding you only get USB 3 if you connect a device to your client machine and pass that to the guest. You don't get USB 3 speeds directly from the server.
http://www.tinkertry.com/esxi-5-1-sort-of-supports-usb-3-0/
Also there's all sorts of other reasons to pass a whole USB 3 card through including allowing you to use USB devices which ESXi won't pass through because it doesn't support that type of device. Or if you've already passed through a piece of hardware you can no longer add or remove USB devices to that VM without shutting it off.
Ultimately it works fine in 5.0. They haven't added server side USB 3 speeds from ESXi 5.1 and they've broken the whole USB controller passthrough. So this isn't a step forwards but a huge step back.
I'm just glad they've added support for Server 2012 to ESXi 5.0 so I can keep using that for a bit longer. Hopefully they will eventually fix this problem.