Oldfan said:

I completely disagree. Why would I need them to spoil everything with dialogue that ruins all the plot points for me?

A trailer should give you a heads up heading into the film about what you are going to see:

1. Who is the protagonist / who are the protagonists?

2. What is the protagonist(s) goal?

3. What villains or obstacles might stand in their way?

For instance, I have seen that new Mars trailer with Matt Damon several times this season. I haven't seen the film, but I know that if I do see it, I will be watching a movie with Matt Damon's character being the main character, and he will be stuck on Mars and people will be trying to rescue him. Whatever else happens in the film, I have no idea, and I have no idea whether he will live or die or whatever larger point there might be to all of it, so it's a nice mix of setting up the audience but not giving away too much.

This Star Wars Trailer on the other hand tells me:

Explosions! Storm Troopers! Lightsabers! Jedi! Harrison Ford is back!

It's Star Wars!

You see how underwhelming that looks to anyone who isn't drunk on kool-aid?