Because, if you are flying along, and hear the engine grenade (which they seem to be doing with increasing frequency) which generally means the gas generator has locked solid. Hitting the starter at point will be like shorting out the battery. You stand to heat (red hot) any of those heavy duty cables that run between the battery, solenoids, and starter. So now you have a nice burning smell to accompany you in your glide. If the engine puked oil, you stand a moderate chance of lighting that off as well. To say nothing of losing your battery, and thus nav/com. Other than that, yeah, no problem.
Now, if it is showing
any Ng, then go for it, you have little to lose. If it went out with a bang, and Ng went right to zero, you have a lot more to lose. An engine that has not locked will show 5% or more in the glide.