If paper wastage is a concern, you could argue that you shouldn't shoot 35mm film either. 24x36mm negatives don't fit traditional sizes of photographic paper proportionately. You have to crop the image, or waste some of the paper.
I like square negatives. They don't suit everything (when they don't and I'm using my Bronica SQ-A I shoot accordingly and crop) but I find that square negatives fit some subjects better. When I do use the whole negative, I have considerably more real estate than I would if I used a 6x4.5 camera (6x6 cm square is 1.78 times as large as 4.5 cm square, after all). When I don't, I'm no worse off than the 6x4.5 guy is. I use a little more film, but 120 film is not that costly so it's not a huge issue.
I imagine that if I find I shoot a lot more rectangular 120 images than square, that I might consider getting a 6x7 camera, but so far I haven't found myself all that disadvantaged for not having one.