I know this is an OLD thread, but I just had to reply.
I understand what you are saying with flat cables having a "separation" between individual wires, preventing crosstalk, interference, etc. I also understand how rounded cables could be a problem, as the individual wires are bunched close together.
Still, when you have a flat EIDE cable in your system, people tend to bunch them up, fold them, stick them close to other cables like power cables, and so forth. Unless the flat cable is perfectly straight and doesn't come into contact with other wires (hard to do I know), wouldn't a flat cable have the same "problems" as a rounded cable does?
Just a thought.