I have seen a ton of articles on the interwebs in the past week or two debating over players who disregard fluff by taking oddly color models and representing them as something else, i.e. blue blood angels or red salamanders. All of the articles outline both sides of the argument, namely the fluff bunnies who think that ultramarines have to be painted blue, the emperor is still on the golden throne, and the storyline of 40k is the standard of measure for our hobby efforts, and the gamers who think that fluff shouldn't matter in regards to how you paint or play your army.
I think the answer to this is found in the purpose of fluff. Why does GW give back stories to flesh out the various players in the games they create? Well, it is obviously to sell more models. You don't sell nearly as many plastic men when they don't have names, backgrounds, honors won, and chiefly connection to the player. That is also what separates this from board games. They aren't a hobby. Warhammer is.
It is my belief that the purpose of fluff is to serve as the fertile soil for getting gamers creatively involved in the hobby. You are more likely to nicely paint and continue playing this game if it connects to you. The backstory of 40k, its myriad heroes and epic conflicts serve to get you involved in the universe. The game itself is the medium for taking part in that universe's goings on.
So, luckily, since there are more and less creative people out there, the fluff accomodates both. For those less creative who need a stringent set of guidelines to go by, GW has created fluffy, well structured armies such as the better known Marine chapters (Ultras, Blood Angels, Black Templars, Grey Knights, etc.) . For those of us more creative players, there are the second foundings, DiY chapters, xenos armies, etc.
So, which is better?
Neither. You can choose to play Smurfs and adhere to the spirit of their chapter in how you create your armies and play them or you can create a mythical DiY offshoot chapter that uses whichever rules you identify with best. I think it is up to each player to find the army that they best identify with and use that army. Armies and personalities really do match up. You know what I mean. The half crazed, pit stained, sweat soaked behemoth of a player is going to be playing an Ork army. It just happens that way.
Do you need fluff to play Warhammer?
Certainly not. As a matter of fact, I know plenty of people who don't even enjoy the hobby aspect of the game, but love the competitive, tactical aspect. I think it ultimately makes the experience more full and rich, but it is simply a creative aid. Whether an individual chooses to treat it as foundation and gospel or a simple set of guidelines by which create their own interactive fantasy/sci-fi environment is up to them.
I do think that respect needs to be a priority, though. Should a player value fluff, they should not look down on other players who don't. The opposite is also true. I don't assemble, convert, and paint my models to appease others and neither do they, but I can respect a person's individual perspective on the game as long as they are willing to play by the rules and be a good sport.
A final thought goes out to competitive events that award points or prizes based on fluff and hobby, otherwise known as soft scores. I am of a simple opinion, namely that you aren't forced to go. Most tournaments and competitive events post their requirements and structures ahead of time, so if you don't like their qualifying criteria, don't go. If that is okay with you and you think you can work within the system and still keep a competitive edge, go. It's that simple.
So, fluff is a great vehicle for helping players/hobbyists to immerse themselves into Warhammer. It helps build creative ideas that help us to own and produce quality models and armies. It drives our desire to play and win. However, it isn't gospel truth. The company that produces the official fluff changes their mind on a regular basis, so it can't bear that much weight. So, if you love the fluff, stick by it. If you want to create your own, go for it. Fluff is just that, fluff.