I'm honestly curious as to what kind of serious work is being done that requires a full 10 inch?
I use my iPad to review and edit documents and spreadsheet. While that might be doable on a mini, the larger screen allows me to see more of my document at once, which is always an advantage. People working with PDFs will probably want a 10 inch screen. Not a serious use, but comic books and magazines display almost full size on a 10 inch screen. Landscape keyboard on the 10 inch iPad is almost the same size as a full-size keyboard.
Yesterday, I had no work to do, and my iPad did end up staying in my bag all day while I read and texted on my iPhone 5. (The screen on the 5 is a great improvement on the 4/4S, not because of the increased size, but because the color is so much more clearer and the screen really pops at you.) But if I have any work to do, my iPad comes out. And I'm typing this reply now on my iPad, but I wouldn't even attempt this on my iPhone -- I'd just wait until I could get out my iPad or get to a computer. Whether it would be comfortable to type longer posts like this on the mini, I wouldn't know until I give it a try. But IMO, larger screen size does make tablets more useful.
I have a 13 inch MBA, which I do use when I have a desk to prop it up on. But I find it too big to use on my lap in places where I have no flat surface to put it down. Now the 11 inch? I don't know -- I haven't tried the 11 inch other than the few minutes I play with display models in stores. But the point is, for the type and amount of work I do while on the go, the iPad is enough, and I don't really find myself motivated enough to buy a 11 inch MBA. Other people might indeed find that they'd rather carry a MBA 11 than an iPad. People have different needs, and different devices or combination of devices fit those needs.