Two reasons for having multiple readers:
1. When my mom runs out of books and I give her my EZ Reader, she feels guilty about taking it away from me, even though I'm fine reading on my iPod Touch or laptop for a while. If/when I get an additional e-ink reader, it won't bother her to borrow one of them now and then.
2. I like shiny new toys!

Unfortunately (or fortunately, depending on your point of view) finances prevent me from feeding the addiction as much as I would like.