My iPad 4 IS my laptop. I use it for office programs (pages, numbers, keynote), the small amount of photo editing I do (snapseed, iPhoto) and mostly movie/video watching either at home or on trips. My mini is my gaming, browsing, reading device.
I like to read and I like to watch movies. The iPad mini does reading very well, while the iPad 4 does movies better.
I haven't owned a laptop for a few years now.
I can see why some think owning both is superfluous - could I get along without one of them? Sure I could. My iPhone takes care of plenty (albeit on a smaller screen than I would like for some tasks - reading being the primary). But I like being able to use my iPad as a "cable-box" in my man-cave. I can airplay watchESPN to a monitor and have the game on while I'm playing Call of Duty - or I can have two games on at the same time.
I too have begun to use my mini more though - the iPad 4 just feels huge after using the mini....but, I wouldn't get rid of either at this point. I purchased the 4 (after selling my 3 - overall cost me $17 out of pocket) and I was given the mini as a gift. Why sell one (which has depreciated in value) when I can keep it for a long time and always have a backup?
I don't plan on upgrading the regular sized iPad often like I have in the past - the mini will get the yearly update, the iPhone gets the 2 year and the iPad will be relegated to a "blow the socks off me" update.