Shootist is right. In other words, what happens is that the max transmission data rate of any one USB controller is shared among all connected devices. 'Devices' being numerous, Bluetooth, iSight/Facetime camera, card reader, internal and external keyboards, trackpad, audio, and peripheral 3rd-party gear as per your liking such as storage devices, etc.
Check your Mac's System Report, the USB controllers are listed there, including what's connected to them. You'll see that the "internal" devices are spread out over the controllers via "internal" hubs. You'll see what the max speed is of both controllers and individual devices - left in marketing terms with "up to".
Aside from other potential bottlenecks, you want to connect external devices which are supposed to run as close to their max speed as possible to a controller that has no other demanding devices aleady connected (e.g. keyboard, or the unused camera).
Admittedly, guessing how Apple would treat additional USB ports, if they existed - giving them their own controller, or connecting them via an existing internal hub - is pure speculation; though it's clear that controllers cost money, and we know that e.g. PCs with four ports typically use hubs for at least two of them...
Fact is you have only two USB ports with individual controllers now, and you could indeed benefit from utilizing a TB port for USB as well since it'd also use its own controller (again, other bottlenecks elsewhere aside). Then, though, the question is, why convert TB to USB instead of using TB gear e.g. for storage? For now this may make the bottleneck in the wallet all too obvious, but if I were you, I'd rather get a cheap USB hub now and save my bucks for the TB drive until its price tag fits me... they're cheaper by the hour...
