i think e-books are very useful.
1) they are much less expensive. one can download most e-books for free, and even if one does have to pay, it is usually a fraction of the cost of a paperback. my income is low and it was e-books that allowed me to continue reading more or less what i wanted while at university and afterward. i wouldn't have been able to buy all the books i needed in real-time format. a paperback is 1/15 of my monthly salary on average, more if it was printed in a foreign country.
i realize that going to a library could be an option, but most libraries here have an exceptionally poor range of books (read: if you need something, it is not going to be there). when i was a student there was a constant fight over the sources available at the department library - there was one or two copies of each and whoever came first would get them, while the others had to order theirs online for discount bookstores, search for e-books or buy paperbacks, if they could afford any. the national and town libraries had nothing to offer that would be of value.
not to mention that, for someone like myself, reading anything at a library can be difficult. i grow restless almost at once and can't concentrate - i'm not sure why, perhaps because it is not "home" (as in, a familiar, secure space) and there is not enough physical privacy, what with people i don't know walking around all the time, or because i cannot do what i do in private to help myself focus (change my physical posture to a more comfortable one, stroke one of my cats, make some tea etc.). i have to get up, walk along the corridor and back and go to the coffee machine and then return to the reading. besides, going to a library requires time, which i frequently don't have - whereas with an ebook, i can open it on my pc and read it in fragments while doing something else, such as translating or housework. this matches my haphazard style of reading perfectly and there is no time lost. finally, there are books one would want to possess and to be able to use them whenever it becomes necessary - books on philosophy, poetry collections, and, for me, textbooks, grammar reference sources, collections of articles on linguistics etc. reading any of those at a library once a week would not be enough.
2) there is a far greater chance of finding rare or specialized literature in e-book format. i don't think i would be able to locate skorik's grammar of the chukchi language any other way - i doubt it is available as a paperback even in moscow, and here it is practically unheard of. but i was able to download the ebook from a russian public e-library without any trouble. so far, i've found only one real-time hungarian textbook, which is excellent but does not cover all the morphological aspects of the language, but online i found another two-part textbook that is more extensive, as well as a comprehensive grammar.
3) when i move house, i won't have to have boxfuls of books transported to the new apartment (again, saves a considerable amount of time, effort and money). and if i have to go abroad, whether permanently or not, i will be able to take my library with me rather than leave it behind.
the only exception are books that are complex enough and easier to read in small sections of several sentences or paragraphs which i would then want to reflect on. i do prefer those to be paperbacks (bought a copy of heidegger's "being and time" a few weeks ago for this reason).