"Does this give a true picture as to the actual capability of the PC, or is it just a gimmick?"
If you click the little link on that page that says "Re-run the assessment" you'll see each how each hardware component is tested. So, no, I don't think it's a gimmick as it has its uses. One scenario is if you're in a shop looking for a new computer, you've got your eye on three different models but aren't sure which one's the most powerful - Win 7's Performance Index should be able to help you make that decision.
It's also a useful tool in showing you which hardware components are slowing down the PC's overall performance - with that info you may want to upgrade the PC with faster RAM, or a faster hard drive etc.