There is no absolute reason to have more than one. In most cases, we are gadget fans and can't help ourselves, so then find excuses to have more than one. The exception is that some larger units work better for technical, craft and cooking pdfs and comic books.
I currently have 5 units.
Nook Glow - has all bookclub reads on it. Just bought this about a month ago. I bought it to replace the Nook ST that I desperately missed after giving to sibling.
Kindle 6" - This is my takealong that I keep in my purse. Has a large variety of books, magazines and anthologies. Only cost $50, so if lost, stolen or broken, I won't be too emotional about it.
Kindle keyboard - I have hundreds of books on here, all sorted by series.
Nook HD+ - Primarily used for craft and cooking PDFs, audiobooks and 'in bed' reading. Can be set to scroll and has TTS. Too heavy for regular reading, IMHO.
Kindle HDX - Bought primarily for whispersync titles and TTS. Too heavy for regular reading IMHO.
Kindle Keyboard 2 - bro no longer wanted. Has all my emagazines (like Asimov, Ellery Queen) on here so it gets used regularly.
One of the problems with having multiple readers is that you have to regularly use all of them or the battery just goes bad. My bro and I bought our Kindle Keyboards at the same time. Mine was used daily and his only the first couple of months and then not at all. My battery lasts WAY WAY longer than his does.