The reason USM is destructive is you have little control, so the best thing is to turn it off and use a method where you have complete control.
The artefacts produced by a poor scanning methodology are passed on down the chain so it you create halo's, noise, aliasing (false grain) or other issues you're stuck with them.
The correct method is to supply your image 'digital negative' with as much information from each RGB channel as possible, no sharpening auto exposure etc.
The putting on USM doesn't give you more detail in your scan it just gives you more noise.
To answer why switching of USM would that be a truer rendition is simple; less noise you add to the original signal the better.
True to what? The original negative/slide, print conventionally to understand what you are being true to.
I was told by a scan expert that most flatbeds scanning 35mm can't resolve grain on medium speed films, so if you see grain you have to ask yourself 'what is that' its noise called aliasing.
It should be easy to get natural looking scans if you give your photo editor the right basic file, of course digitising film changes it, but with the right workflow it should look very natural-and sharp!
Barney by
Photo Utopia, on Flickr
But hey, if you use USM in the scan module and are happy who am I to argue?