May be were off the point of the thread a bit. As far as I'm concerned, replay format compatibility is essential- both in terms of Mono/Stereo/Surround and also tonally, which is important too.
That's one reason for Far and Near Field monitoring in a mix area.
This isn't really an M/S issue but is a question that affects all mic arrays, A/B, M/S, cross pair and so on.
To clarify my point in respect of M/S, the better the match of the mic capsules, the better the replay format compatibility will be.
Dave Blackham
UK