Anyhow. . . the assertion was that 4e makes 3x books "obsolete" or "worthless," neither of which is true. As has been stated, you can still use your 3x books to play D&D 3x or as fluff sourcebooks for 4e. Plenty of people do both.
True, within certain limits.
I can use my 3e
Oathbound book no problem. Just like using a 2e book, it's mostly flavor anyway. The few mechanical elements have fairly smooth translations (prestige classes become paragon paths, prestige races perhaps become feats).
Using my 3e
Tome of Horrors is more problematic. Unlike the 2e MM's, the 3e monster books had a lot of stats and abilities in them. I can translate them, but it takes work, and I'll probably loose some of the detail that the ToH originally put in.
My 3e
Savage Species book is fairly worthless.
My 3e
Tome of Magic book also isn't going to be doing anyone any good in 4e.
My 3e
Complete Divine book doesn't work so well, either. Though that does have a 4e update of sorts in
Divine Power, the books don't really cover the same territory.
My DM resources will be easier to use than player resources, in general. Still, even some of those are a little broken. My 3e
Draconomicon isn't very useful, though I'm getting two books I can buy to update it.
It's also a little harder using my 2e books in 4e. For example, Angels and Archons and Devas and Aasimar are so different, any of my 2e Planescape books that reference conflicts with celestial beings is going to require major DM investment in making these things make sense again.
But certainly 4e doesn't invalidate 3e's and 2e's entire library. I do see it as tougher to integrate with older stuff, though, largely because of how drastically meanings changed in 4e.
In 2e to 3e, a gnome was a gnome was a gnome. Halflings had a bit of a facelift (though they still had "Hobbit" roots, it was assumed adventuring halflings weren't hobbits), but elves were the same, and angels were the same, and demons were the same, and dragons were the same, etc., etc.
3e to 4e, we now have a new concept of what
a dragon is. That doesn't invalidate everything 2e and 3e ever wrote about dragons, but it certainly makes it more difficult to integrate.