Okay - let me be more specific here, since you already mention
"should not be attempted until the skill developed away from traffic. In addition, coasting with the engine off is illegal in some areas."
The reason why it is illegal in some states (and other countries) certainly has to do with the fact that in case of an emergency power-assisted braking (besides other safety systems) is not available with the engine turned off, thus increasing stopping time and distance TREMENDOUSLY, potentially causing an impact that could have been avoided otherwise (not even talking about potential injuries and damages). Car manufacturers work hard to maximize safety by increasing stopping power which can be measured for example in meters or yards from 60-0 and much of it relies on power assisted breaking as well as power-assisted
ABS systems (which enables to steer AND swerve while braking with maximum force). As is the case with almost any emergency, they are mostly unforeseeable. I do not recommend to use this technique other than in the wide open without traffic - it is simply compromising too many safety measures of most cars (i.e. power steering, ABS, servo assisted braking, ESP, to name the most common). Besides, causing an accident due to self-induced lack of braking power is certainly an issue for insurers! I think point 55 (engine off version) is potentially the most unsafe/dangerous here. Rather be safe than sorry!