I use my full size iPad for lots of photo/image review and processing. I also use it to present to clients--so it's vital to me to have the full size device. I also find that the speed bumps with each update make a difference for me so I tend to upgrade it every year.
With all that said, I also have an iPad mini, which I mostly use for reading/web browsing at home. The size is simply more comfortable, particularly for use in bed. With that said, the lack of a Retina display bugs me, primarily because I use it for things where that is particularly noticeable.
I'm planning on upgrading to a rMini in the near future, but likely won't upgrade it regularly like I do the full size iPad. For reading and web browsing the A7 chip will suffice for quite some time. I may even throw in some light gaming on it from time to time as well.
So, if you really want a justification for both, that's mine. I can see getting 3-4 years out of a rMini, while I'll upgrade the Air every year like I always have. So in the grand scheme of things, the mini isn't really a particularly large expense.