I think these discussion always miss this:
How the heck could a 12 year old that had never played before (or 16 year old, or whatever) develop ANY fluff at all that had any meaning? I just think the MM should have:
A clear description of the monster
A little bit about tactics
A little bit about the culture. What is the difference between the various evil humanoid races?
As for "most DMs want their own fluff". I call bull. I want fluff I can use, or not use. But, I want fluff to inspire my ideas. I have two kids, golf, ski, volunteer for two groups, work full time.....I don't have time to do all my own work. I do have time to take decent to good modules and make them fit into my concepts (but my concepts are based on 40+ years of reading and watching movies and playing D&D).
Fluff is needed for beginners. It is needed to provide inspiration. And, sometimes, it is needed to be used as is.
As this came up again just now:
I think this assumption is totally wrong. It could only be considered true when one thinks of a 12 year old as a blank slate.
But all kids have been exposed to media and stories by then, in one way or another and already have developed a fantastic worldview based upon what they have been exposed to.
Ok, I guess that´ll be Pokemon and Dragonball, and so on, but these images is what they intepret stuff with.
I think it´s pointless then to give such a person fluff based on Howard, Leiber or Lovecraft, when they haven´t been exposed to this and probably never will be, even if this fluff is what we older players may consider to be superior.