"I have an iPad 3 and a Nexus 7. And my wife also has an iPad 3." Completely makes sense to purchase an iPad mini then......
I assume you're being sarcastic, but I'll answer this anyway because I'm in nearly the same situation.
My wife has an iPad 2, I've got (or had) an iPad 3 LTE and a Nexus 7. I can sell all three devices and have enough money to purchase two iPad minis (one with LTE). Now, the reasoning is sound. The wife wants a smaller/lighter tablet because she reads on hers about 75% of the time so the size advantage is paramount.
My own reasons are a little more complicated. Yes, I'd like to have both devices (iPad 3 and Nexus 7), but I get so sick of carrying two devices. I ultimately only used the Nexus 7 for listening to podcasts at night in my office and for reading e-books. It wasn't good enough for web browsing when compared to the iPad and there are a ton more games on the iPad as well. So I needed both devices. The Mini w/ LTE is a good compromise...I can play all my games, I can browse the web ALMOST as good as a regular iPad and significantly better than the 7, and I can read comfortably (even better than the 7 from a size/weight standpoint). So it makes sense for me to downsize my collection of devices and get a Mini. It's the perfect middle ground as far as I'm concerned.
So to conclude, for some people, Minis are the answer.