The only real advantage I can see over the ipad 4 is one-handed use for an extended period of time. The iphone 5 or most other smartphones do a much better job at this use case as this is how they were designed to be used all the time. You can make the argument that one is a tablet and one is a smartphone, but in reality, if you're holding your Mini with one hand, you are really limited to using it like a big smartphone.
I guess many people here use it primarily as a one-handed consumption device (causal browsing, tv shows, ebooks), but I can just turn to my iphone for this use case while enjoying all the advantages that full-sized ipad 4 offers in every other scenario.