Found this great gem from The Loop:
“But while XY microphone recording is the most obvious method, it’s not the only game in town. The Mid-Side (MS) microphone technique sounds a bit more complex, but it offers some dramatic advantages over standard coincident miking. If you’ve never heard of MS recording, or you’ve been afraid to try it, you’re missing a powerful secret weapon in your recording arsenal.”
Honestly, never used Mid-Side recording. Since I have a Rode NT4 (XY Mic) I never have the need. However, this does sound interesting.