Could you explain why is it so fundamental for the dialog tracks to have not a single hint of sound effects in it ? What's the problem if at some point, a cloth rustling is in there ?
The point is not that the clothing rustle is there, but the fact that it is missing.
The idea behind an M&E mix is that when the film is dubbed in a language other than the original no other sound work should need to be done.
Let's go with a specific example.
In the scene Tony cocks his pistol and puts it to Steve's head.
It was decided that the pistol cock recorded during production would be used in the final mix. If the sound edit was done correctly the pistol cock was placed on a sound effects track (as Greg/APE mentioned).
However, if the pistol cock remains on the dialog track
there is no pistol cock in the M&E mix.
This one is fairly obvious, but a huge percentage of indie folk edit and mix sound themselves, and don't know proper audio post protocols, so the pistol cock remains on the DX track. It would have to be replaced along with the dubbed dialog, and the whole point of an M&E is that nothing but the dialog needs to be dubbed. Any sound effects that are missing from the mix will need to be replaced, increasing the cost of the dubbing process, infuriating the foreign distributor, and generally placing you on everyones sh!t list.
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Now we are getting into the more "scientific" and practical aspects of film sound.
I have mentioned often that your brain possesses an "editing and mixing" function. There are sounds that fill your world, and most of them are not "useful" to you most of the time, so your brain "mixes them lower" or "edits them out" so they do not distract you from what is important to you at the moment. It's a film cliché when a character says "It's too quiet." Yet the absence of a sound is a warning to our most primal selves - everything is not normal, so our fight or flight response awareness kicks up a notch. You're not paranoid or adrenaline-filled, but something is wrong so pay just a little more attention.
In the scene Tony cocks his pistol and puts it to Steve's head.
What other sounds are in this little example? There is Tony's jacket rustling when he cocks the pistol, but, more importantly, Tony's jacket rustles when he extends his arm and points the pistol at Steve's head. From a purely practical POV when someone extends their arm ("It's over there") your brain edits out/mixes down this very quiet, subtle sound. But when it comes to film sound if this clothing rustle is missing your primal "something is wrong" instinct kicks in. Now, obviously, seeing a pistol pointed at another human beings head does not fit with everyday life, but the missing cloth rustle will pull the viewer out of the illusion created by the filmmaker.
When it comes to M&Es missing details (like cloth) leave the dialog disconnected from the rest of the sound track, which will, of course, pull the viewer out of the film.
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The artistic side.......
Now, from a sound design point of view, having the clothing rustle may be extremely important to the tension of the scene. If Steve's tied up in a chair and watching Tony cock and aim the pistol his fight or flight instincts are at an extremely high level, and his senses are kicked into overdrive, so the director and sound team may make the cloth rustle very prominent in the mix to emphasize this fact.