I love that they went this route.
It not only gives people something to talk about, but it gives almost nothing away from the movie, since even the people who lived or died off-screen during Infinity War is something that we will likely find out about at the beginning (unless we think they are dead and just are missing ;D).
It's like the Days of Future Past cover where you see the characters and your looking at all the characters like, "Oh shit, now I have to read this to see what happened to so-and-so!"
It also gives Marvel a chance to show all the characters who may or may not be in the movie (although I'm guess they all will be in some way), without having to put everyone in the trailer just to remind people who exists in this universe and who may or may not have been dusted, and instead just focus on what the core of the story is about. Infinity War got to have that type of marketing, about getting everyone together, this gets to focus on what is at stake. It's really cool, and I really like that we've seen so little of the film and it's over 3 hours long! We are going to be in for a treat!