First, I don't think I'd dismiss that as "fluff." There is already so much OGC out there that the actual mechanics of most spells and feats are not particularly meaningful anymore.
As the market continues to grow you will see more and more people wanting cohesive combinations of "crunchy" bits - not necessarily complete adventures and fully fleshed out world settings, but feats, spells, classes, etc. that work well together to model a specific setting, era, etc. (Like Avalanche Press’s Black Flag, Judge Dredd, Forbidden Kingdoms, etc.)
Additionally, the OGL and the d20 licenses are pretty clear and very specific. The OGL defines the following as product identity:
"...artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs..."
That's certainly not all "fluff" no matter how you look at it. If you choose to also follow the d20 license, you must adhere to additional restrictions. However, they are also quite well spelled out in the SRD Guide and deal only with the actual core mechanisms of the d20 system - creating and advancing characters - and defined terms that you can use but not modify or extend.
Finally, if you choose to also make use of or modify portions of the SRD, then that by definition is already open. However I don’t think that from the above documents you can make the case that any spell, for example, is then necessarily derivative and so open content. Even for the Skills section, which has already been released, what is OGC is the format and the mechanics – not the actual characteristics and description of the skill itself (except for the specific ones they have also decided to make OGC).
The purpose of these licenses taken together is very simple – to allow widespread use of the d20 system, but to also keep the core free from adulteration. The SRD is an addition to that to make it particularly easy for people to create works that expand upon the D&D genre, but without infringing on what WoTC considers their most valuable content – or restricting the ability of others to develop their own.