Stik,
I think DubTrub has the answer. In an single engine aircraft, lose the prop and the Cof G moves back a bit. Lose the engine and prop and it moves back a lot and is unflyable.
Twins are not affected as much because they have the engine roughly in line with the C of G so an engine detaching itself would make a lot less difference. I suppose there is twice the chance of it happening.
I did hear of a cable that could be looped between engine and fuselage. The idea was that if the engine came loose because of a prop blade failure the engine would still be attached and keep the C of G in flyable limits.
Its a sobering thought when you weigh the prop (I just re installed mine after overhaul) and think that it all whirls round at 2500 rpm connected to the noisy bit by a 1.5 inch diam tube with 150+ HP going though it.
The only good thing is that I don't have enough spare brain cells to think/worry about it when I'm doing aeros!!