I am using a black border on my Holga prints because they're rough, show process, thereby re-enforcing the sense that a photograph is not reality and because it's a useful exersize while becoming acustommed to shooting a square negative.
A lot of the historical reasoning behind using the black frame has always seemed silly to me. The developers of cameras and film formats are not the last word on the best presentation of an image. I crop whenever cropping serves the image. I cannot always place my camera in the best spot...nor do I always have the most appropriate lens...and sometimes the shape of the frame or sheet is just irrelevant to the subject matter and reduces its impact.
All that said, it's useful to learn to shoot to the format. It's an artificial constraint but like writing poetry in a difficult meter, developing the skill has value. Breaking a rule you're incapable of abiding by is not the same as breaking one you've mastered.
As for the subject of "verification." It's such a fallacious conceit. It indicates that the negative wasn't cropped in the printing, sure...but as for demonstrating something about objectivity or truth, crap.
Photography is not objective. The editorial process is at work every time you do or don't release the shutter...every time you decide how you will select an aperture to control depth of field...and every time you position your camera, deciding what's in frame and what's not. As far as I'm concerned, composing a shot to keep something out of frame is no more or less honest than positioning the blades of your easel to accomplish then same thing.
When I use a black border it has to do with aesthetics, not ethics. People that use it to indicate something about their ethics are either being disengenuous, ignorant or cynical.