It's impractical, but there's no reason why you cant. It's really just another key you can press.
The key is reaction times. Most people can tap a key or click an extra mouse button way faster than they can push a pedal down with their foot. The only non-driving practical use I can really see for it is as a push-to-talk key for voice chat. Keeps your hands focused on your hands instead of trying to do hand gymnastics to play while holding down CTRL or Shift or whatever. Still expensive even for that, I wouldn't personally buy one and use it for that unless I was also already planning to use it for driving games.