Instructors who employ surprise engine stops via surreptitious cutting of the fuel flow may flatter themselves that they're increasing the 'realism' of the exercise. But in my experience this is an illusion, and often they're simply massaging their own ego, providing little or no training value.
For one thing, there are many ways (some of them quite subtle and insidious) in which an engine can cease to function, partially or fully. So focusing on this one scenario, generally presented once only in a training course, as the "true" example is counter-productive.
The purpose of the deliberate engine shut-down in a ME course, done at altitude and briefed properly, is to demonstrate (and give the student confidence) that the airplane can be flown safely home on one engine, not primarily to provide an opportunity to practice emergency drills.
Further, the airplane doesn't fall out of control just because an engine has stopped - it does so when uncommanded yaw is either not recognized, nor contained effectively. Learning how to deal with uncommanded yaw/undesireable aircraft state/unusual attitude/loss of stability is the first priority. Only subsequently can, or should, the "why" be properly analyzed.
Per Big Pistons Forever:
In Canada the requirement to do an actual engine shutdown/feather/restart was removed from the ME training requirements after a safety audit showed that this practice contributed to numerous actual engine failures/failed restarts and several accidents.
Similarly with the removal of spin training from the FAA - or CAA as it then was - private pilot syllabus in 1949 (!). After that the incidence of stall/spin accidents decreased substantially. The training had become more dangerous than the contingency it was supposedly addressing.