This is true, but I would add that, as I've pointed out before, it's not "the trailer," it's one of the trailers. No film has just one trailer anymore. We've already seen a teaser trailer, a US trailer, and an international trailer, and they've all been different from each other. We will inevitably see more, and each trailer will be skewed to emphasize different aspects of the movie, because that is the routine formula used by the small number of marketing firms that make most movie trailers these days.
So we need to stop thinking in terms of "the trailer." Each trailer needs to be understood as just one step in a larger campaign. The teaser and the first full trailer are bound to be deliberately incomplete in their presentation of the film, because some things are being saved for later in the trailer campaign.
There's a lot of truth, but can I throw out some nuance... each trailer can build the buzz for or against a movie.
The first trailer is the snowball at the top of the hill, and can build more momentum, like the first Star Wars trailer.
Also, are there nay examples of wildly different trailers/ads that helped a movie? The only thing I can think of is Deadpool (was anyone fulled by the romantic movie posters?)
Good trailers can create a good opening for a movie -- the resulting reviews that either crash it, or keep it going.
The way I see it, it can breakdown like this (blanking out on examples for most):
A good trailer can represent the substance of a film, if it's a good/enjoyable film. So Star Wars the Force Awakens had points that touched fans....but the film was filled with that. So I think people would say the successful trailer represented the film.
A good trailer can also get people excited about a film that might not actually be that good, but get people interested enough to check it out (I am thinking Skyline)
It's possible to have a bad trailer for a good movie, and that's just disappointing.
And bad trailers for bad movies... well that's a dime a dozen (literally)
Also, a good trailer can get people excited for other trailers, or at least alert people of different interests, who might pay attention when they get "their" trailer. Deadpool seems like an excellent example.