Rosa, on Feb 16 2009, 11:00 AM, said:
Sir Wright made a comment on the DVD to the effect of, "Swan Lake is Siegfried's story, but the ballerina's ballet."
Wonderful comment !!!

Depends on whether you consider the "story" most significant -- or the "ballerina." Wright apparently decided to focus on Siegfried's back story. (Set to Odette's music, however. See below.)
If you focus the audience's attention on Siegfried, there's not really a need for a Prologue. The first scene in effect BECOMES the prologue to the most significant experience in his life.
Rudolph Nureyev famously complained that Siegfried has "nothing to do" in the ballet. He then set about in the 60s refocusing the audience's attention to the "Siegried story," adding solos, switching music around, etc. Later (for Paris, iin the 80's) he merged Siegfried's Tutor with Rothbart. This is interesting and enjoyable as a variant. But it's not the Swan Lake most of us want to see again and again.
I prefer to focus on Wright's claim that SL is "the ballerina's ballet." If you agree with Wright, it makes sense to use Odette's musical theme (or the suggestion of her theme) in the "Introduction" as the opportunity for showing us her back story.
So what about Act I (i)? No one on stage even knows that Odette exists. It's been suggested that having a Prologue takes away the sense of the Prince's birthday party which follows. I don't agree. No one on stage knows about Odette, but WE know. Our awareness of her imprisonment hovers over the scene and infuses our experience of it. It adds texture and emotional depth to Siegfried's discontent, which he feels but cannot understand. Later, in the lakeside scene, Siegried will learn the story from Odette herself. This is a revelation for him, but a confirmation for us.
Great drama is full of instances like this, when the audience is "in on the secret" long before the protagonist finds it out. Those of us who know the SL story by heart don't need a Prologue. We already have the information. But what about those who don't have it? Maybe they do need a little extra visual assistance.