And it makes it easier to confuse the film with an unexposed one, which is great for double exposure experimentation. 😀
I think the 'leader out' lobby started many many years ago when various film processing practices advocated pulling the film back out of the cassette and onto a reel in the darkroom. It made it easier for amateurs to do. In all my years I have never ever met a professional who would do anything other than wind the film all the way back in, not only to guard against double exposure, but because it is one less trip through a felt light trap that may or may not have grit or dust trapped in it.
As for the M3, yes sometimes I remember mine stopping dead and needing to pull the take up spool out with the cassette, but this was not a regular thing and usually the film went all the way back in easily.
Steve