I've never heard this and think it is a little funny. You guys know the shutter blades move at the same speed for 4 second photos as they do for 1/4000 of a second right? It is the gap, or time in between the blades moving that determines the shutter speed.
It's Canon *Rumor* for a reason, right :-p ?
I've picked up the "high shutter speed" theory along the way, and as you note several people did so without anyone objecting yet. The theory as far as I remember is that the shutter blades stop more sudden at higher shutter speeds, meaning the exact difference of your statement that's it's the same speed in any case.
I know that the shutter mechanism works differently on different models, on Rebels the shutter movement is coupled to the mirror while on xxd+ it's independent. It would be nice to get more input about this, because I'm actually trying to avoid the highest shutter speeds for this reason.