Sometimes watching a preview for a highly anticipated film can be more enjoyable than the actual film that follows.
In the case of Dark Shadows, your assertion would aptly apply.
As for talking during trailers, it's a faux pas as far as I'm concerned, but I'm willing to tolerate it as long as the comments are terse and uttered sotto voce.
One of the many reasons why I avoid cinemas all together.
-Jamman