I only have one a prs-350. I do have a small Samsung tablet but I don't consider that an ereader, its used solely for mobile web browsing. My girlfriend also has a reader a prs 650.
As for those that have more than not just one, but 4 or 5 or 10(not that there is anything wrong with it) I think its a mixture of many things.....impulse buying, addiction to ereaders/tech, feeling the need to have to upgrade with each need edition, and just plain like buying new stuff.
I mean I know many try to rationalize why they need 10 ereaders, but really do you really need 10?lol Probably not but to each their own. Heck I probably...no I don't even need one. I could happily go back to paper books if my 350 broke. Buts its the novelty of the ereader that I enjoy. But it is definitely not a necessity.
Now I may get another eventually. My 350 is two years old now. But unless I catch a really good deal... (like the staples $30 off coupon and an additional $5 that people were using to get THE nook simple touch for 1.50 that I missed out on
).....I can wait. Afterall I have had buying addictions of my own over the years and I pretty much have learned my lesson.