Huh? Not sure if we're talking about the same thing (we might be). I'm not saying that 3rd Party publishers didn't use the OGL license, but how many of them used significant OGC content from other companies works? Granted, I didn't scour the section 15's of all the books I own, but I really doubt it's significant . . . . or am I way off base here as you seem to imply?
I'm certainly too lazy to head out to the garage, unbox some of that stuff, and start looking . . . .
I think it depends on publishers. Some were liberal with stuff, but others didn't use anything but the core SRD and their own rules.
At first, I personally swallowed the OGL concept and really believed in it, and I wanted to support stuff that was 100% OGL, and reference many works in one main adventure.
Practically speaking, however, using a lot of OGL sources has a catch-22 effect. Ideally, for economic reasons, you'd probably want to just reference content in short form "stat blocks", and the OGC section would hopefully lead to more sales for the creator. But I can see from the consumer end, they might resent having to buy 3-6 books to find out the details. And if you end up including the whole shebang, like put the full monster stats in the appendix, then you're sort of leeching more from the original publisher, and discouraging the person from purchasing the original source.
From what products I purchased, I noticed very little re-use of 3pp content outside of the SRD, with the exception of two items.
1) Creature Catalog and Creature Catalog 2, which were the first big OGL products from a major publisher, so they were first to market.
2) Tome of Horrors I--which pretty much IMO did not have to do with Necromancer's own brand, but simply because it was an authorized collection of monsters that we all knew, created by E-Gansta Gygax and Da You Kay Crew (from Fiend Folio), with assistance from TeaSaR posse. In other words, the product mostly benefitted from the D&D legacy rather than anything new.
Beyond those two, I didn't see many, maybe one or two isolated references in individual products.