What about math, English, and history need to be up-to-date? I went to an excellent high school, and we had plenty of books from the 70s. And no freaking iPads or digitized textbooks.
On my digital textbook of a 70's book, I can highlight text for review later, using one of several colors (so I can classify notes), and can attach a text note to the highlight. I can do this with a physical book, of course, but it takes more time (to highlight and to write text by hand) and unless I'm doing it in a separate notebook, I'm also destroying the book for other users (not to mention the trees it took to produce the book, the mercury the paper plant dumped into the river, etc).
In the digital copy, I can also view all the notes for an entire book all together rather than paging through 600 pages, and can print those notes or copy/paste the notes between my iDevices and my computer. Those books and all the notes I make therein are automatically synchronized between all my iDevices and iBooks on the Mac, so I literally have my entire library with me wherever I go... at work, at home, on the train, waiting at the DMV, sitting in another class, in the park, at the beach, on vacation, etc. And that iDevice weighs a heck of a lot less than just about any textbook, and because I can store hundreds or even thousands of textbooks on one device, they are always available and I don't need a truck to move them from place to place.
Within a given book, I can search the entire book for a word or phrase. I can share selected text via email, text message, social media, etc. And I can define a word and find synonyms just by holding my finger on a word I'm unfamiliar with. I can watch videos or listen to historic audio such as the 1969 moon landing or a re-creation of the Gettysburg Address. I can interact with 3d models. I can navigate through maps. I can take interactive quizzes. I can take end-of-chapter practice tests to make sure I learned what I was supposed to learn.
And while content updates are more common in some subjects vs others, nearly all subjects can use them:
Math: While formulas may not change that often, there are new theorems and proofs coming out all the time. And more importantly, the WAY math is taught changes rather fluidly. A digital book isn't just about displaying a formula, it's about interactively showing the student how to understand what's happening as that formula is worked. Imagine: Instead of showing a student an example of a graphed rational function, give them an iBook that lets them change the factors and the formula his/herself, to see the effect on the graph interactively. Can't do that with a 1970 math textbook.
English: Try learning grammar using printed rules in a textbook, then pop open an app that interactively teaches you grammar and quizzes you afterwards. And grammar rules do change over time, and there are certainly plenty of new words added to the lexicon each year. A 40-year-old uses dialect and vocabulary that is somewhat out of touch with the present day.
History: There are revisions, new theories, news of archaeological digs and new discoveries, etc. all the time. Imagine popping up a video of a greek trireme being excavated, including an interactive map of where it originated, where it ended up, and a depiction of what a typical fleet of ships would look like. What about the ship that was just uncovered under the World Trade Center? That would make an interesting side gallery next to a story of the 9/11 terrorist attacks. How many years would it take that to get into a printed book (if ever, since the cost of printing photos would probably outweigh the benefit of including it)?
Who the heck would want a printed book anymore? ;-)
Seriously, pick up a digital textbook such as Eo Wilson's Life on Earth (free) to see what the technology can do. It's a much more immersive learning environment, and studies have proven that the challenges are far outweighed by the benefits.