I started digiscrapping with rectangle size simply b/c I knew I wasn't gonna print a lot of my pages and wanted to use them as desktop and screensaver for my computer. But I did encounter all the problems Rachel mentioned above, plus:
* a lot of element packs come in square sizes, too: overlays, page borders, stitches, ... (it's not only the grunged up paper edges that get lost)
* you sometimes might get less views/comments in the galleries, b/c people just don't like your differing page size.
* templates however are not such a big deal, b/c what I used to do is just copy the clusters over to my re-sized canvas and then work with that.
Last summer I switched over to 12x-12x (for a photo book project) and found it was a lot easier to work with everything, but honestly, I kind of really miss the old ways, b/c I like it when my pages show up on my computer, really fitting my screen. So, I wish the designers would put out more rectangular shapes - I'd probably totally go back to that format.
If you're still considering, you can check out my
gallery (go back to page 16 and beyond - I got tons of pages to give you an idea of what opportunities you have with this size).