Late 3.5 consisted of the second run of splatbooks, Eberron setting books, and a bunch of a combination of testing thigns out for 4e and throwing experimental stuff against the wall and hoping it stuck (PH2, Incarnum, Dragon Magic, Tome of Magic, Tome of Battle, Heroes of X books). All of which was very fringe material, but no cheaper to produce then the first run of splatbooks and the core setting books for FR. Pretending those things didn't sell because they were a product of WotC's "B-team" rather than because they were inherently fringe concepts is silly.
Well, too bad there's no way we can pull up a list of books by year. Oh wait...
* Fiendish Codex I: Hordes of the Abyss James Jacobs, Erik Mona June 13, 2006
* Tome of Battle. Richard Baker, Matt Sernett, Frank Brunner. August 2006
* Dragon Magic Owen K.C. Stephens, Rodney Thompson September 12, 2006
* Cityscape Ari Marmell, C.A. Suleiman November 7, 2006
* Fiendish Codex II: Tyrants of the Nine Hells Robin D. Laws, Robert J. Schwalb December 12, 2006
* Complete Scoundrel: A Player's Guide to Trickery and Ingenuity Mike McArtor and F. Wesley Schneider January 16, 2007
* Dungeonscape: An Essential Guide to Dungeon Adventuring Jason Bulmahn, Rich Burlew February 13, 2007
* Magic Item Compendium Andy Collins, Mike Mearls, Stephen Schubert, Eytan Bernstein, Frank Brunner, John Snead, Owen K. C. Stephens March 13, 2007
* Drow of the Underdark Robert J. Schwalb, Anthony Pryor, Greg A. Vaughan May 8, 2007
* Complete Champion: A Player's Guide to Divine Heroes Ed Stark, Chris Thomasson, Rhiannon Louve, Ari Marmell, Gary Astleford June 5, 2007
* Exemplars of Evil: Deadly Foes to Vex Your Heroes Robert J. Schwalb, Eytan Bernstein, Creighton Broadhurst, Steve Kenson, Kolja Raven Liquette, Allen Rausch September 18, 2007
* Rules Compendium Chris Sims October 2007
* Dungeon Survival Guide Bill Slavicsek, Christopher Perkins October 23, 2007
* Elder Evils Robert J. Schwalb December 18, 2007
Of that list, only
Tome of Battle,
Dragon Magic,
Dungeon Survival Guide,
Magic Item Compendium, and
Rules Compendium were done by the main staff, and three of those five were compilations.
So 5 of the 14 are (partly) done by staff. But two are half freelancers' work as well. 9 of the 14 (64%) are done by pure freelancers, including most of the non-compiatoon books in 2007.