I have read some of Christie's books again after obviously knowing the solution to the mystery. Does it ruin my enjoyment of her books because I now know, well not necessarily.
However when you read something or watch something a 2nd time it's always going to be a different experience than the first time. You are not going to get that first experience ever again and I would rather have my first experience remain unspoiled.
When you know something you can go back and read (or watch) it again and look at it from a new perspective. However for a first time reader/viewer I want my experience to be entirely fresh. I have been spoiled for things and have not been spoiled for things. And while I can still enjoy something after being spoiled if it is well written I know I would enjoy it far more if I went into it not knowing.