I have one Kindle (an old Kindle keyboard) that I use once a month to read a KOLL book on. I re-registered it on Prime account strictly for that purpose. My Paperwhite, my (grown) daughter's K Touch, and my daughter-in-law's Kindle keyboard are a shared Kindle account. Our family practices trickle down Kindle-nomics! I'll probably get the new Kindle Paperwhite and one of the K Keyboards will be donated to the troops.
I tend to give away my replaced kindles almost before I stop needing them. I just love getting them into the hands of a reader. Most of those people remain on my account, as they then have access to a large library of kindle books. I sometimes wonder whether I should really keep a backup, but the fun of sharing usually overrides that.