The complications here are flash capacity, battery life, weight and sun light screen readability.

There is no doubt that lots of people who own an iPhone and an iPod of some sort, where they'd ideally just have the iPhone. With the iPhone currently stuck at 32GB, it "forces" some people to get an iPod as well.

And some people probably own the iPad and Kindle, rather than just the iPad because of the screen. While the iPad screen is often readable in sun light in the UK, I imagine in many other parts of the world it generally isn't.

Then you have the weight issue. For the sort of person who wants to commute with their eBook reader, it might make perfect sense to commute with the Kindle, but have the iPad at home (because of course it can do a great deal that the Kindle can't).

For me personally, the iPhone/iPad/Laptop cover all bases. My iPhone is always with me and is use for smartphone stuff, some reading, music* and podcasts. The iPad** is the primary non-work browsing device and my laptop is what I sit in front of for many hours a day working.

Certainly there is some overlap between the three, but they all serve distinct purposes for me and it is hard to remember how I survived without them *blush*

I expect most people starting from scratch with no devices would choose to buy less devices than they ended up with, there are plenty of people around who have iPods simply because they owned them before they had iPhones or people with Kindles who bought them before the iPad was around.

* I really need a 64GB iPhone to fit all my music on, but not enough to make feel the need to start carrying an iPod as well
** to be fair the iPhone could do everything I use the iPad for, it is just that the iPad does some of them better due to its size when you don't need pocket-ability

P.S. I didn't actually buy the iPad myself, but if it hadn't been bought for me I expect I'd have bought one with my own money anyway