Chuck, I don't like to be at odds with you because I respect your work a great deal, but... Yes, if you know what you're after and can't frame it correctly because of the aspect ratio of the camera it makes sense to crop. If you're shooting on the street and simply can't get to where you want to be in time it sometimes makes sense to crop. I do it more often than I'd like. And you're right: looking beyond the viewfinder borders is a very good thing, but it's something you should do before you frame the picture. Having seen your work, I'd be willing to bet you rarely crop unless it's to correct an aspect ratio.
But all of these exceptions assume you know exactly what you're after when you trip the shutter. I think HCB was right that cropping to save a picture you screwed up because your attention wandered is going to fail, unless in the end you're willing to accept a sow's ear and call it a silk purse.
I don't buy the idea that you can improve somebody else's picture by cropping, or teach somebody composition by cropping. That's the other person's picture, not yours. You can't see what he saw, and you certainly can't see what was outside the viewfinder. You learn composition by studying the work of the masters. There simply isn't any other way.
As far as tonality and color are concerned, I often see color and even tonality with which I can disagree, and that's a different thing. Cropping deals with composition and geometry, the image that made you want to trip the shutter. Color and tonality are choices that often can be made after the fact.