I've got a small scratch on my lens. Is my lens worthless?

Not at all. Even on a wide angle lens it isn't likely that most people will see any change to their images. 

It is possible that you may "see" the problem area with a wide angle lens when focusing very close with small apertures, as on some designs it's more than possible to bring the front element nearly into the focus area, but still, I'd judge this to be a rare occurrence. The longer the focal length of the lens, the less likely you'll see any tangible image quality issue from a small scratch. 

Mostly the scratch is just slightly reducing contrast.

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