Think about your stereotypical super powers. What if somebody had them but only partially? I don’t mean lame powers like in Mystery Men or The Specials; I mean the really good powers, but not having them at full strength. I can’t imagine that would be terribly effective at fighting crime. Imagine someone who is able to fly, for example. Who’s to say they could necessarily fly any faster than they could walk? It’s funny to imagine somebody slowly puttering across the sky in a spandex suit, while the criminals make their getaway in a car.
Then there’s the ability to turn yourself invisible. Well, that’s another situation in which partial ability doesn’t do you much good. The ability to become translucent is likely to aid in medical diagnosis, but it’s not really handy when it comes to fighting crime, although some villain might become squeamish at the sight of your internal organs and run away.
The only partial-strength superpower I think would be really useful is super strength. You might not be able to arm-wrestle Superman, but you’d still be able to impress the ladies at the local gym. And it would be even better if you were really scrawny.