Leica is sticking with 3:2 because... they where the first that suggested it!
There is no reason behind aspect ratios, it was the film industry that suggested them and the cameras where (off course) made depending on what was available from the film industry...
Digital sensors follow what was the most popular aspect ratios with film. There is an (obvious) reason behind that... To preserve the already developed habits so that the new media wouldn't require different approach from the user coming from film.
There is no better or worst aspect ratio... There can only be what suits each individual best.. There is no standard anymore for print sizes... with roll paper printers (that are the standard for fine art prints) one can print whatever aspect ratio he likes without wasting any paper at all...
There are actually technical reasons for why Leica chose 3:2 for the S; for starters they wanted to minimize the flange distance as much as possible for greater freedom in lens design and compatability, and that meant having a shorter mirror box. And what do you need for that? Yeah, a shorter mirror.
The closer the format is to square, the greater the angular travel of the mirror, which increases vibrations and reduces shot speed... It's just simple geometry. A 3mm difference might not seem like much, but it's about 10% of the plane height, which bears a multiplicative effect once physics come into play.
Having used an S on a few occasions and being an owner of a 645Z, I can tell that there is a difference in the amount of recoil produced by each camera during exposure; the S is smoother and easier to hold at lower shutter speeds, although both have quite a bit less vibrations than a Phase SLR.
