Stormtroopers being present in the movie does have to do with the plot...
It means the Empire still exists in some form or other, so we now know something about the conflict. It isn't a major spoiler, and I don't think anyone ever said it was, but it's a spoiler nonetheless. If a picture of Spiderman being filmed on the set of an Avenger's movie was leaked, it would be a spoiler. The existence of Spiderman in the Avenger's universe and his reveal would be spoiled. In this case, the existence of the Empire and stormtroopers (and this Empire is able to afford chrome troopers, indicating they're still a strong power) spoils that reveal and the fact that we're still going to be seeing conflict with the Empire.
The same goes for the pictures of Sith we've seen. They're undeniably spoilers; the only question is how significant they are.