How important is low light performance really?

[WhoDo]

As an avid reader of all things photography related, I've noticed the industry seems to move in different directions according to phases the buying public are experiencing. In the digital era things like sensor size, megapixels, and image quality (IQ) have all received plenty of attention. Lately the buzz seems to revolve around low light performance, with ridiculously high ISO figures being touted for different brands.

With the recent release of the Nikon D600 and and Canon 6D bringing full frame sensors within the reach of the masses (well most of them anyway), aficionado's of each brand have been keen to identify points of difference between these new monsters of the enthusiast and consumer markets. The most commonly cited difference is the stark variance in ISO range for each camera - the Nikon maxes out at 6400 while the Canon offers 25,600 (I think?).

With that background information, I've been reading reports from Photokina (you haven't been living under a rock somewhere, have you?). The one covering Leica I found particularly enthralling, but it left me with a question. Here is one of the most respected brands in the photographic world, able to boast devotees among the world's most respected professional photographers, and their top of the line offering (the medium format S series @ $26k+) has a maximum ISO of 1600! Yep, only 2 zeros! Why? Some will say, "It's a studio camera". Sure, but think hard - how often do any of us really venture too far beyond ISO 800 anyway?

I know there are some applications that require higher ISO, low light, low noise performance - event photography, indoor sports, bird photography where ISO is used to boost shutter speed, etc. Generally, though, who really needs more than about ISO 6400 in ANY application? And wouldn't ISO 1600 be perfectly usable for 99.99% of photographers anyway?

Before the Canon crowd notice my sig line and suggest I'm making excuses for Nikon, let me state this is NOT about Nikon vs. Canon, even if the original impetus for the discussion came from that debate. I'm really interested to hear why we need the ever-escalating emphasis on ISO's in the 10's of thousands? Let the opinion's flow, but if you make any assertions I would really appreciate supporting citations if you have them. Game on!