The problem with teaching down in foul and back in fair is it doesn't work. So why waste time on it. Throw a few more rounds of bp, or hit some ground balls......something worthy of time.
Because whenever a runner gets picked it's usually because he's not paying attention. His sudden awareness that there is a play on him leaves him no time to complete the circle.
A runner who performs the down in foul back in fair is not even going to draw a throw. Because he's not napping. If you consider that "working" so be it.
And, to say the down in foul back in fair causes a bad throw, is wrong also. The throw may have been perfect. It just hit the runner. If you're going to throw you have to throw it there or why throw.
I won't teach my catcher anything but the perfect throw.
The long and short of it is, runners get picked due to inattention or an overly aggressive gambling style lead. In either case, down in foul and back in fair does not apply. If he's got too big of a lead, or caught napping, the last thing the runner will do (hopefully) is round off his return the 3rd which slows him down.
I loved rounded runners.