The most rational argument I've seen for 6x7 is that a lot of pro's are shooting for magazines and stock libraries (which ultimatly end up in magazines and the like). In these cases the rectangular format (ideally 6x9 but 6x7 is more practical) allows more of the negative to be used in the final image. 6x6 would need to be cropped to fit a magazine page.

If you're aiming to produce work for your own benefit, and to sell as artifacts in themselves then stay with what works for you.