Most publishers create ebooks in all formats figuring it doesn't matter whether mobi is more important than EPUB or if the content is read on an iPad more frequently than on a mobile phone. That approach means these publishers have no idea how their content is being consumed. It also means they probably don't have a direct channel to their customers or some other way of polling them on their preferences.
At O'Reilly we like to stay on top of our customer reading habits and preferences. We monitor device and format trends through surveys and download statistics (from our direct sales channel). For example, here's a chart showing which primary and additional devices our customers read our books on:
As you can see, a computer is the O'Reilly customer's preferred reading device and the Kindle family is a distant second. What I find interesting here is the fact that Android tablets are much more popular reading devices for O'Reilly content than an iPad is. In fact, for our customers the small-screen iPhone/iPod combo is also a much more popular reading device than the iPad. Another interesting tidbit is that the iPad's popularity is almost exclusively as a second option, not the primary reading device.
Now let's look at preferred formats:
Here we see PDF still dominates; we learned long ago that most of the reading taking place on the computer is with PDFs, not EPUB or mobi files. This is a trend we've seen for years now and PDF doesn't seem to be any closer to relinquishing its format leadership status now than it was back in 2009, for example. And despite the Kindle's popularity EPUB is preferred much more so than mobi.
That begs the question: If the Kindle is such an important device for O'Reilly customers (see first chart), why is mobi a distant 3rd in format popularity? Is it possible our customers are loading their Kindles with PDFs? Sounds like a great question we need to add to our survey...
These charts reflect the preferences of the O'Reilly customer. Unless you also happen to publish technology books I'm pretty sure your results will look different from ours. But are you even taking the time to ask your customers these questions?