M-S does not allow you to choose polar pattern in post.
what it does do is give you the ability to adjust the amount of presence (vai gain) of the forward facing (mid) mic and the width (side) of the figure of 8.
The side info is more ambient info where as the mid is forward signal.
Kicks ass on stage!
It gained innital popularity as it is very easy to mix in post to a mono signal for broadcasting purposes.
When I run it, I tend to like a lot of side information. If you are in a not so good local, like the TS in a large indoor arena, it can sound like ass. if you give it too much mid (forward) gain, then you get an uninvolving mono sounding crap recording.
I've got examples of great and crappy M-S recordings if anyone wants one.
many preamps will do the M-S mixing internaly (if you want them to) and then it outputs an XY signal. Personaly, I like doing it "on the fly" this way. You just need Etymotic earphones so you can hear what you are doing, and then its a pice of cake.
the V2/3 does a great job at it.