Even in portrait mode, iPad only shows one page at a time. I love writing on my 27" iMac with two pages showing at once. I think perhaps the iPad could work for getting rough drafts out, and being able to "jot down" ideas that come at you anywhere you happen to be is a definite plus for tablets. But once I start editing, I find I tend to fling lots of paragraphs around, so for that, the bigger the screen, the better.
Again, it's all personal preferences and working styles, and I always enjoy hearing about other users whose habits are different from mine. And I do think you are right that a Bluetooth keyboards makes it much more convenient to write on the iPad. I do have an Apple Bluetooth keyboard, which at first I thought I'd use with my iPad. But I found that for me, the hassle of carrying around the keyboard outweighed its usefulness. It's another piece of equipment to carry, and I was always second guessing myself whether I needed to bring it or not. I considered keyboard cases, but the keyboard is in my way when I don't need it -- which, for me, is about 90% of the time. I ended up getting an Air for when I need a keyboard, and am happy to use my iPad entirely without a physical keyboard.