In my opinion, cropping is a visual tool, in the same category as burning or dodging. It can be overused, but it can also make an image stronger. I don't see it as an automatic sign of poor capture technique or poor visualization (although it CAN be indicative of those things, just as having to make tons of tonal adjustments in printing CAN be, but isn't necessarily so). I like to print big, so I try to fill my frame with the subject to the extent that I can. I can't always do it (don't always have the right lens, physical constraints, blah, blah, blah) and there are times when I'll visualize an image as being square and having only a 2:3 camera, I'll make the shot knowing that I intend to crop it. I agree with the posters who have pointed out that edges are important and how we read them has a strong influence on the overall impact of a photo. If my approach bugs you from a philosophical standpoint, that's fine with me. Not everyone is going to like my work (cropped or not).
ANY technique can be overused or applied badly, resulting in a boring show or one I dislike. When I hang a show (or when I look at one), it doesn't bother me at all to have pieces in different formats if the overall visual effect works.
Finally, this argument (the one about whether or not cropping is a mortal sin) is as old as the hills. I've been hearing it for as long as I've been talking to other photographers, and I doubt if anyone is ever going to agree on much of anything .