If the mirror could be removed without any serious disadvantages to creative ease, I'd be in favour of a square format. One would have to rely upon an electronic viewfinder in place of the optical, but these have improved considerably over the years.
One of the main attractions of the Canon 50D was its 920,000 pixel LCD LiveView screen. If I could press my eye to an optical viewfider which produced a 1mp image, I think that might be sufficient (but I'm not sure, of course).
The idea behind the square sensor is simply a more efficient utilisation of the physical camera shape and size. Having dispensed with the mirror, a 5D2 instead of being a 21mp camera with a 3:2 aspect ratio, becomes a 32.5mp camera with a square aspect ratio, but retains the option of the 21mp 3:2 aspect ratio whenever that's considered appropriate.
In fact, one could take this a step further and have a dedicated button on the camera (like Michael's MLU button ), but this button, with each press, would change aspect ratio from 16:9 to 3:2 to 4:3 to 5:4 to 1:1 etc.
With such a camera, even the very common 4:3 aspect ratio image would have a greater pixel count than 21mp.
Apart from the disavantages of not having a true optical viewfinder, the only other concerns I can think of are the increased vignetting in the extreme corners of a square format using current Canon lenses, since such corners would intrude to a greater extent into the lens image circle, and the increased demands on battery life.