Sticking to the OP question no, I don't justify having both iPads (and less if you have an iPhone). I have a 3, got a mini as a gift for my brother and iPhone 5 and an iPod touch.
Was very reluctant of the mini initially I've been playing with it for the last 2 weeks, have to admit that the form factor is great, a lot more portable than the 3 and all, but yes lack of retina is something I notice, still for taking notes, reading, pdf marking and such I prefer the 3 a lot more since I can cover more of the screen also while web browsing (I find myself zooming a lot more in the mini) I still can carry the 3 on my purse or backpack with no problem since I'm not on the go all the time.
Besides both devices are basically iPads, it's not justified from my point of view, get one or the other and put that extra money somewhere else.
But I can see myself in the future getting a mini (specially if it comes with retina) when my settings change and I have to be back running from one place to another.