It's really a question that can only be answered by the individual.
Personally, I have had an iPad Mini as a companion device ever since they came out. The mini was my go-to iPad for just about everything and the full-sized iPad was the specialty device. I even tried to reduce the foot-print of the full-size iPad to 16 GB Wifi with the iPad Air 2 (I had a 128 LTE iPad Air previously), but it didn't work and I was glad to give it up when the Pro came out. And yes, I have the iPhone 6+, and No it doesn't even begin to be a replacement for the mini.
So is it overkill? I don't think so, but each person has to decide that for themselves.
If you really just want it for reading/surfing, and you think another iPad might be overkill, there are other options - you could get a low-cost/priced Android tablet. The Kindle Fire is $50, and there are a host of other cheap Android tablets. Depending on your needs it might suit you (of course if you think you want it for anything else, you have to think about what it would take to balance two different Operating Systems - your mileage may vary for the cost/benefit analysis).
That's just me though... My list of tablets/devices is... Not small (iPad Pro, iPad Mini 4, Surface Pro 3, Samsung Galaxy Tab Nook, iPhone 6+ x2 [work/personal]... And, of course, my non-tablet Retina MacBook Pro)(also not counting any old, not-in-use tablets/devices). I gather this is not abnormal for this forum though. =)
(I'll also acknowledge that, though I love the Pro and it does travel most places with me, I could make room in my life for an Air-sized tablet if it had Pencil support... I know, I know... I need help.)