It is important to keep several points in mind when considering the purpose of the Chorus. The Chorus speaks twice in the play, before the beginning of Act I and before Act II. It functions as a commentator on the action and basic meaning of the play. It sets the scene in Verona, Italy, and summarizes the action. Traditionally, choruses served as the voice of public opinion in Greek tragedies, often reviewing the plot and offering moral judgments on the participants. Here, the Chorus seems to reserve the judgment, while still explaining the plot for the audience.
It is significant that the plot of Romeo and Juliet was taken from an earlier version of the story. The theme appeared in the fourth century in a Greek tale and later in the sixteenth century. In the later version, the city is Verona, and it was the first to call the hero and heroine Romeo and Giulietta. Probably Shakespeare’s most direct source was a long English narrative poem written in 1562 by Arthur Brooke, called The Tragicall Historye of Romeus and Juliet. Shakespeare used the characters in Brooke’s poem but developed them in much greater depth and detail, thus transforming the story of star-crossed lovers into the most famous love story ever known.
So Shakespeare didn't invent this story. His contemporaries knew the basic story, and some may have read the longer poem. Thus, the Chorus wouldn't be reealing anything they didn't already know. Indeed, as his play-goers were coming to see a tragedy, they would pretty much know how it was going to end. The only new bit of detail would be the length (hence the "two hours traffic of our stage") and perhaps the setting. Also, since the orginal basis was a Greek tale, he may have been attempting to conserve the traditional format.
Finally, I don't think the Chorus ruins the story at all. By now, practically everyone who's familiar with popular American or British culture knows the story of Romeo & Juliet by the time they're 10, and thus the deaths come as no surprises. Even modern audiences have an idea that a tragedy will end in death for many characters. The beauty of this play lies not in knowing how it ends, but in following the journey of the lovers through their brief but passionate affair. The Chorus does not tell us of the fights, the tears, the vows of love that come along the way-and that is where the true meaning of the play is found.