I've previously made a fool of myself by saying why I don't like M/S mic technique, so I may as well do it again.
If the Mid mic is an omni, outside of the near field, then the theory works quite well. The Mid and Side figure 8 mic can be summed to create strong Sum and Difference signals that yield an accurate Left/Right stereo picture.
What you get is an ambiant recording, with a stereo spread that sums to nothing when summed to mono, leaving only the Mid mic. If the Mid mic is fairly ambient there isn't a huge difference between stereo and mono.
But it seems far more popular to use a close cardioid inside the near field for the Mid mic, and still call this M/S mic technique. What you get here is basically a dry Mid signal, and a wet Side signal that sums to nothing when mono'd. So there is a huge variation between hearing this mix in stereo and mono - tracks that should be ambiant in the distance become dry and jump forward, destroying the mix relationship.
If you don't care about mono compatibility, it doesn't matter. But if you want things to sound right when mono'd, I think it matters a lot.
I suppose panning closer towards the centre means there is less of a rude shock when mono'd ...
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