IMO, cropping for reasons other than the necessity of fitting an aspect ratio represents a failure to capture the image properly in the first place.
Obviously, if an image works best cropped square and your camera is 4:5, you will need to crop. But if you have to do more than crop the :5 down to :4, you screwed up the capture.
There seems to be a lot of confusion here, Jonathan. First, all images are cropped. It's not possible for a photographer to take an image without cropping it. He crops it first through choice of camera and lens (in relation to the scene), and crops it again, if required, during processing.
There's nothing wrong in revisiting a shot months or years later and cropping it yet again in accordance with an improved or different personal aesthetic.
Being limited in vantage points to shoot from is no different than being limited by the native aspect ratio of the camera in hand. You get as close as you can to what you really want, so you minimize the amount of negative/RAW you have to discard in post.
This is not always true. Getting as close as you can changes the perspective. In some shots you might want to stand back for the sake of the different perspective. If you don't have the appropriate telephoto lens, then there might be no other option than to crop in post processing, in order to achieve the desired perspective.
Now, I understand with the Canon 1Ds you will probably have a great incentive to get as close as possible to the imagined composition, because a mere 11 million, rather outdated pixels don't allow for much flexibility. If you decide the composition requires a square format, no matter how close you get, you're restricted to a 7.3 mp image, and any further cropping will reduce that number.
Those IDs pixels may be large, but they don't compare with a modern small pixel, such as the ones on the Nikon D810. In all respect, the D810 pixels are either equal to, or better than the IDs pixels. Something for you to consider.