If I was a pro or a commercial photographer I'm sure I'd be cropping my images left, right and centre as different forces come into play.
Somewhere online I saw an example of a pro's crop, some random fashion shoot that looks like it was shot on his couch, and when I saw the original I thought, "I would burn that neg out of embarrassment." Then, when you see the crop, you're like, yep, that's a full-page glossy ad, right there, no retouch necessary, crap coffee table and funky curtains gone, and the lighting is brilliant.
So, as an amateur it made me think, OK, how many decent pictures am I passing up because I wouldn't bother to consider the crop? But I'm totally right there on the other side of the fence, all of my most proud pictures (regardless of format) are ones where they would not be improved by cropping, and when I look at pictures I admire, I love seeing the borders, vignetting, and all the rest of those tiny aspects that let you peek at the effort that went into the image and, to me, help bring that effort to life, that remind us that as much as there was the scene there was an observer who was skilled and fortunate enough to capture it.