Have I read them?
Completely?
No. But I need too. I'v played in 1st, 2nd, Skills & Powers, 3rd & now 3.5. I've played dozens of other RPG's, some similar to D&D, some not.
This means I get things confused from time-to-time. And Since I'm the DM, this can lead to all sorts of DM-Player conflicts. Plus, I hate to have to stop in the middle of the game, furiously flip through books for something, somewhere; or make a DM command line decision, only to find out later I was completly wrong & a player got screwed because of it.
Thus, I'm trying to read the core books cover to cover, for several reasons.
1. Help me get my thoughts straight (I have several rules sets floating in my skull & occasionally they cross-pollenate). Reading the book cover to cover helps me keep 3.5 seperate from the other voices in my head.
2. Know where things are. I'm not trying to memorize every bit on the Core Rules; I just want to know where to look when somthing come up. Reading it helps with that.
3. Know the rules. How can you run a game if you've never read the rules? The best way to learn to play might just be jumping in, but then you pick up the bias of the DM's you play under.
4. I spent 100 bucks on that. If I'm not even going to READ it, why buy it?
As for the other books, I buy them, but only read them when I need them. For example, my Wed game is about to spend several months traveling the Frozen South, so I'm now reading Frostburn cover-to-cover.