One reason to get the MBA11/13 is portability. But why wouldn't you just get an ipad AND a MBP15. You wouldn't bring the the MBA13 with you every time anyway unlike the ipad and you can't just flash it in public as easily as an ipad so its function is limited to doing pure computer related stuff. Plus for the occasions that you do bring it with you, how much of a hassle for that rare occasion is it to bring the MBP15 with you. For home situation its really a no brainer too. Moving the MBP15 from your desk to the toilet to the bed is just as easy as moving the MBA11/13.
Or, OR I can just get an MBA 13... For anything I would actually use an iPad for out in the wild, my iphone would handle far more efficiently. A short email is easier to write on my iphone, i can just pull it out of my pocket and generate a quick reply. A long email is easier to do on my MBA. So the iPad for me is good for neither short or long emails. Any type of content creation is easier for me to do on the MBA over the ipad as well.
As to your other points. Laws of physics > your opinion. And MBP 15 is not just as easy to move around as an MBA. It's bigger, heavier, fatter and has a larger footprint. Especially true for people like me who find the ipad rather useless and actually take their MBA to places other than the toilet.