The books and DDI each provide some distinct benefits:
DDI is accessible anyplace you have internet access, making it possible to have almost all necessary resource material available from a laptop. WItht he books, you need to lug around all of those books to have access to all of that information.
The books will be yours forever. DDI will someday disappear. If you want to play 4E in 2030 (as opposed to 7E), you'll likely need the books. Actually, DDI compendiums and character builders disappear as soon as you stop paying for them - you're renting access, not buying it.
DDI makes it easier to access things you know exist. The books are easier to thumb through to find new and interesting stuff.
Books have pictures. DDI doesn't. Both can be seen as advantageous.
DDI is incomplete, while the books are out of date.
In the end, both products are good things to have.