Okay, I consider myself pretty much of a pedant where the rules of the game are concerned. I am no RPG industry insider, just a player and DM who happens to like the game and (I think) know it fairly well.
I went and re-read DB's first post. I think he presents a fair point. For reasons of plot or story, DMs and designers of modules and games may want to introduce creatures with unique and special abilities. However, if these abilities go against the rules as written, does it mean that they cannot be used? No, only that they have to be used carefully.
Here's an rough analogy. Let's say I go to see a movie called "Vampire". The protagonist faces a humanoid creature with prominent fangs that drains blood from its victims, changes into a bat and a cloud of gas at various points in the story, does not reflect in a mirror, and summons a pack of wolves to attack the protagonist. In the climactic encounter, the protagonist presents a holy symbol, and the creature knocks it aside with a sneer. He produces a clove of garlic, and the creature only laughs. He stabs it in the heart with a stake, but it plucks it out and continues attacking. The sun rises, but the creature is unaffected. Finally, the protagonist manages to trap the creature in a cave and sets off an avalanche that buries it, and the movie ends.
Such a movie will not be satisfying to me. You see, the use of the word "vampire" in the title of the movie creates an expectation in my mind of a creature with certain characteristics that I commonly associate with vampires. Instead, what I find is a creature that has some of the characteristics, but not others. What is worse, there is no explanation given why this so-called "vampire" differs so greatly from my expectations. Perhaps it was the intent of the film-maker to challenge our pre-conceived assumptions about vampires. Perhaps he just wanted to portray a vampire as an almost unstoppable force. Regardless of his intent, it ruined my enjoyment of the film.
So, how can a film-maker remain true to his vision while still helping me to enjoy the movie? Simply by acknowledging my pre-existing expectations and telling me that they do not apply in this case. Perhaps the protagonist has the same expectations that I have, but is warned that they are wrong. Perhaps this creature is a vampire that has managed to overcome the typical weaknesses, or perhaps the vampires themselves lie about their weaknesses to mislead those who seek to kill them.
Like movie-goers, DMs who buy a module or players who play in one have certain expectations. One of them is that the rules will be followed, and an explanation will be given if they are not. I don't think that's a lot to ask.