Good post, but I disagree. See people, the majority of people don't go see the movies just because of the "villians" they go to it to see Peter Parker and his adventures. This is why it's called Spider-Man. I think with out the symbiote we could not get into as a complex story about anger, and revenge, and how it can consume a person. From personal life experience, anger can be a lot like the symbiote and can consume a person, and turn them into something they are not. I think without this the audiance would not have had a deep as an experience. I don't think Venom is just the Eddie Brock Venom, I think the story is more interesting with Peter, and his dark side. And how even the good can have a dark side to them. And only the symbiote story could really show this side of Pete.
I think with just the Sandman it would of been a much more 2D plot of revenge and what it can do with a person. I think the symbiote was needed in this.