you're looking at things from the wrong point of view. you're a spider-man fan, for you and i, the Spider-Man 3 title and release date in a 5 second trailer is enough to get me into seats May 4th.
for the general public however, who aren't familiar with the character or movies, in order to get them to fill those seats, they need shots showing the intense drama, like MJ crying, and the insane action, like sandman smashing spidey into the truck.
that doesn't ruin the movie for me in the least bit. because you get NOTHING out of it just seeing a 1 sec shot BUT excitement to see how the characters got to that exact moment and what happened after that moment.
who knows how much less spider-man 1 and spider-man 2 would've made if they didn't show all those scenes in the trailers/clips?