So if you have DDI, you don't need to buy the supplements - like Martial Power, for example?
Edit: No offense taken on the "legal" bit. I never, ever used to think in those terms until I joined a group that plays Living Forgotten Realms. Every other group I've ever played with did whatever the hell it wanted, heh. If we decided everyone started with 20's across the board, they did. If we decided a bastard sword did 1d20 damage, it did. Those are extreme examples, of course.
That's another "depends"...unfortunately!
I'm a DM, and I resolved once I got DDI that I won't be buying any supplements unless they have lots of fluff I want (because "fluff" is not found in the DDI entries).
But a player may want the books: a lot of that "fluff" includes descriptions of the classes and races, cohesive entries about the rules to use the different items, etc.
So it's hard to say. The books are still useful, I'd say, for reference and clearer understanding (as well as organization). But if you ONLY want the stats for Magic Item X, or Power Y, or Class Z, then yeah DDI is fine, and will have it all.
Just to extend the example: books I use to prep for game = DMG (encounter building info is not in DDI, nor are traps and I believe diseases), MM (just to glance through, but otherwise all stats are in the DDI), and PHB (for combat rules reference). I use DDI for magic items, monster stats, and if I need to reference class powers. I would totally pick up the various published adventures in the future, and I use adventures from Dungeon magazine (as well as some of the articles from Dragon...but of course, both these come with a DDI subscription).
I personally won't pick up Draconomicon (the fluff is stuff I feel I can come up with on my own), Martial Power (again, I'm never a player in my group), or even PHB2 or MM2. I might pick up AV2 because there's more than just items in those books, there's also rules for how to use and create them, as well as variant rules and essays on changing up items that I find useful, and that won't be in DDI.