I mostly read in the train, on my PocketPC (five year old Axim still beats the iPhone when it comes to text quality). Taking my PocketPC with me tends to be less hassle than taking even one book (especially if it's a big book). I normally only have a laptop bag with me and books don't fit in there very well, and I can have a book with me even when I don't have a bag with me at all.
Last year's holiday, I ran out of books to read, so I bought a new one using the hotel's Internet connection and started reading it five minutes later, something you can't do with paper books (especially not in Japan where finding stores that sell English books is very hard). I also don't need any shelf space (which, living in Japan, I have very little of).
Those are the reasons I prefer eBooks. As always, however, ymmv.