For my BOB, less is more. I don't want so much stuff that it is encumbering, and a cart would be. A BOB ought to be just what you need to get you out of harm's way and where you can hopefully reorg and take care of business better. It is not meant to be expeditionary.

Most likely I would have to abandon a cart during a bug out situation, especially in an urban environment.

Your philosophy is sound, but methinks a bit mis-applied. If you are looking to securely relocate with as much as possible for extended survival mode, then a cart would be much more desirable, as would a car, or a plane. BOB is one of those "grab and go" items, at least that's how I've viewed it. In Baghdad it was the backpack full of stuff that hung next to my body armor that I would grab on the way out the door during a bunker run, an evac, or any other SHTF event. It was slung over my shoulder on my way out the door of my hooch on the run down the stairs and into the compound, usually while doing up my clothes and armor gear. A cart wouldn't have worked. That is the way I see a BOB deployment anyways. Your results may vary.