Speaking as an author, I *looooove* using OGC, when just writing new stuff would be redundant. But this isn't a matter of re-publishing OGC, it's a matter of bringing into the fold things that add to my own product.
For instance: Tome of Horrors. I was writing something about the Chuul, and wanted to include some potentially associated monsters, and used the ToH's Monstrous Crayfish as something that could be associated with them. But more than that, I built on it...I added templates, gave ways for creatures to summon them, change into them, or treat them as animal companions. I made it particular to the Chuul focus of the product, but I used something from an outside source.
Without the Monstrous Crayfish, there is no doubt my product wouldn't have been as cool. I could've statted up my own monster, but why bother giving a person who owns both the ToH and my product (two fantastic purchases by an obviously discerning buyer.
) the same thing over again, in slightly different form? Why not encourage people to buy things that inspired me, much like a bibliography would?
It's the same way that a product can use Goblins, for instance, as a shared source material. I can use the ToH's monstrous crayfish the same way that anyone has used the MM's goblin. And that adds to the community as a whole, rather than coming up with 30 different kinds of goblin ever could.
It's for the betterment of everyone to use OGC, and to use it abundantly. It betters the game as a whole. It helps foster independance from WotC (which is only going to help the 3rd party). It means that rather than reinterpreting something ad nauseum, you're building on a shared base something greater than there was before. For instance, in the Chuul product, I included an NPC; if some author had my product and was writing a swampy adventure, maybe he'd be inclined to use my Chuul NPC as an emblematic aspect of the world, and maybe the Monstrous Crayfish would appear as an enemy encounter. So then, in everyone who plays that adventure, a shared experience results that is a synthesis of good ideas, and a shared theme, and people in the community begin talking about watching out for wizard-chuul in the same way they now talk about 1e Assassins or something.
It adds to the world, to the community, to the game, and that's something that divided, we won't do, but united, we can (to get a little cliche about it!).