Of course it's pointless. That said, most of us are enthusiasts and will agree that your setup is awesome. If you can afford it why not? There are certain tasks the Mini is better suited for than the Air, and the same applies for all your iDevices. Working out is a big one for me, the Mini fits perfectly onto the dashboard of most machines and is the perfect gym companion. I can watch live TV or stream anything while working out. The Air 2 was too big for that (for me). Mini is much more comfortable to hold (not that the Air is bad at all) in general when just doing basic browsing or reading.
I also have a 13" retina MBP that is my portable work machine and living room couch surfer. No point in me getting the Air 2, I'd rather use the MBP when at home doing general tasks. Same thing with me and iPhone 6s+, it is too big as a phone and too small as a tablet. So my current combo is 13" rMBP, iPhone 6s, Mini 4.
To each his own. I say keep your Mini and enjoy it, it's a great little device that I didn't expect to like as much as I do.