First of all, people's device lists are idiosyncratic. Some list every device they've ever had, even the broken ones, and some list just the one(s) in use. Some sell old devices or pass them along and some hang on to them, out of inertia or as "backup." That is, you can't draw any hard and fast conclusions from device lists.
Then, I think there are two major categories of multiple devices: tech upgrades (a ten-year old reader may work well enough even now, but current devices are much better) and/or degraded hardware, and different use situations. Personally, I do have dust-catchers. I passed along some of my older devices as I upgraded, but not nearly all of them. To an extent, I tried to make use cases for older devices I kept, but mostly it's not worth the bother, especially since I'd then be reading on a worse device.
These leaves me with my five Kobos, which in actuality is mostly four of them. My Aura One is my workhorse, armchair reader. My H2O is my nighttime reader, since the Aura One is a little big for bed. My Glo HD is my purse reader. These are the three I list. I also have a Glo which I use as a book repository, given its SD card slot. I have a couple of hundred books on my current readers, but I can always pick up my Glo if I think of something I own that I bought elsewhere than Kobo. Then, I have a Mini. It used to function as my beach/lake/poolside reader, since sunlight makes it readable and its bulky bezels and OEM case means it can take being tossed around; also, it wouldn't break my heart if something happened to it. But I noticed this past summer that I tended to grab either the Glo or the H2O for the waterside; just a better reading experience. This ties into my comment about making use cases mostly not being worth the bother.
Gadget lust is not as much of a factor for me now as it once was. The incremental tech changes are not sufficiently persuasive. The Clara's not enough better than the Glo HD; the Forma's not enough better than the Aura. I've got a mild hankering for the Libra, where I'd have skipped a generation and where the improvement over the H2O starts to seem worth it. And then I'd make the H2O my new repository, which would pretty much put an end to the Glo's usefulness.
I doubt anyone's still with me, but what I'm essentially saying is that multiple devices make sense to me, but the reality is that only so many (an amount which will vary with persons) actually will see some use, no matter how long one's device list!
