I would disagree. When learning to fly a twin, most emphasis is placed on dealing with an engine failure, and asymetric flight. In a single if you lose your engine you have no alternative but to go down, a twin gives choices, and choices kill. I would say that it is worth the risk to use the mixture to cut an engine, in order to make the situation as real as possible. Besides, the correct procedure for an engine failure at low altitude is basically mixtures forward, throttles forward, props forward, identify, verify, feather with no attemp at re-start etc all the while pitching for the blue line. IF the mixture cable was to break in the short time that it is pulled back / pushed forward then it should be possible to feather the prop and save the situation, most ME aircraft will maintain level flight and possibly climb if not fully loaded. In a single however if the engine was to quit then you really would be up s**t creek without a paddle, so no, using the mixture in a single is a bad idea. Another difference in a single is you do not have to identify a dead engine, if the right throttle is pulled back on a twin, it takes some of the realism out of it. On my ME flight test the examiner pulled the mixture on the take-off roll, no prior warning, to see how I'd react and whether the pre-take off brief "...if I lose an engine before we're airborne, I'll close both throttles and stop straight ahead...." was actually just going in one ear and out the other, or whether I was taking note of what I was saying. It all went ok, so I must have been taking it in...
You could also argue whether it is 'worth the risk' actually stopping an engine in flight during training and the flight test, or whether it is worth doing the 'Vmc demo', just in case an engine fails at the wrong moment, or doesn't re-start. I believe that these risks are worth taking, and indeed nescessary to breed a competent ME pilot.
Cheers
EA