Multi-aspect and why don't we have square sensors?

Panasonic GH2 Leica D Vario-Elmar 14-150mm

One of the things I like about the Panasonic GH cameras is the multi-aspect sensor.

However two thoughts occur to me. Firstly why can't we have an option to use the whole 18MP sensor? Is there some problem why we can't use the corners? If its an "odd" size would anybody care? Or are we going to be constantly resticted to 4x3, 3x2, 16x9 and 1x1?
I have tens of thousands images published either in print or electronically every year and many of these are cropped to fit shapes outside these conventions.

Secondly, why has nobody come up with a square sensor? Is the assumption that nobody would want it? Certainly Hassleblad and Rollieflex had no problems selling their 6x6 film cameras. 

Personally I'd love a square sensor. I use it in the GH2 quite often since I can now get 12MP - 34MB files from the square crop. I don't know if lens design is an issue, but again the 6x6 cameras and lenses seemed to function perfectly well. One great advantage is it saves having to use the camera vertically, which virtually no camera is designed for. Square can also be cropped into all sorts of shapes, and with a large enough sensor it could be seen as more versatile.

Just a thought.