Ok,
...I guess I was lucky when it came to my start playing D&D, as it was my mother's idea after a few people from my elementary school told her I needed a hobby to challange my intellect and creative desires. I've been playing D&D, in one form or another, since Jan. of 1980, which was roughly four months before my 7th birthday.
This was, obviously, during a significant portion of anti-D&D hype, much of which was erroneous urban legend that has as much basis in fact as the world of Conan is an archaeological representation of a previously unknown, real world, culture.
Anyhow, I think having a book aimed at adults is a good thing for the hobby - we are getting older - and just because that book is aimed at older people does not mean it should harm the overall industry, or this particular genre, just because younger players should not see the book.
Why?
Simply put, it has not harmed the movie industry or fictional novels.
Kids read Harry Porter, even after some fundamentalists complained about it being a tutorial for devil worship, or other such nonsense - normally those people wouldn't buy the book anyway, so there is no loss to the market.
In general most anti-D&D hype is based on rumor, myth, and false information - or just plainly ignored facts about the cases people talk about.
I'm sure many of us can remember a certain movie with Tom Hanks, which he doesn't talk about much as it was a rather badly acted part for him, which was based on a real life missing persons case that, in the end, turned out to have nothing to do with the game of D&D and was, instead, just a messed up portion of the life of a person who played D&D. People didn't look at all the facts, but instead looked at what they wanted to see and used that.
Children can't buy R-rated movies, if the store is run properly, without an adults permission - nor can kids under 13 see a PG-13 movie, if the store is run properly.
Those movies are still made, they do not impact the movie industry negatively and, thusly, it should not hurt the hobby anymore then it already does.
People who refer to D&D as satanic, or what not, will already refer to it as such and do not need a book to fuel their fires - they already use incorrect information, false statements, and general stupidity to make those inferences already.
For the record, I'm not calling anyone on this board stupid, I just do not agree with the actions of a certain, large number of fundamentalists who spout of like broken records.
Funny thing is, when you look at both my current D&D groups, we've a health does of various religions; we've a handful of mormans (some jack and some regular), a few catholics, a couple southern baptists, a hedonists, a wiccan, and myself (a Neitzschean Taoist Deist, kinda.

).
Anyhow, more to the point, it is a game and the people who would complain about it, call it evil, or what not would do so without the Book of Vile Darkness being published, so not publishing it for this reason is bad in my opinion.
Children shouldn't see things of an mature nature, so it's good that it has the 'Mature Content' label - Sword and Sorcery Studios has items with nudity and no such labels, we've heard no outcry about that (one item in question is the topless 'wood' elf female in their Scarred Lands DM Screen booklet).
Just my opinion though, no more right or wrong then anyone elses.