I blame Avi Arad for how Spider-Man 3 turned out. Raimi did not want Venom or the symbiote, and he was forced to shove that storyline into his script. I feel so bad for him, because he lost creative control over his movie, and he knew how it was going to turn out. You could see that Sandman and the Harry Osborn/Green Goblin storyline were going to be in the movie originally (they are the best parts of the movie in my opinion). Sandman could have been the best villain of the trilogy if done right, and it really makes me sad because I think Thomas Hayden Church was perfect for the role.
To make things worse, with the script they did have, Raimi had to cut the movie greatly, resulting in so many plot holes. There was a scene filmed with Jameson's son bringing the symbiote to earth, scenes of sandman visiting his daughter's doctor, extended fights in the subway, and an alternate meeting between Venom and Sandman. Heck, there was no reason to cut out the second long clip of Peter seeing himself as Venom in the mirror.
If they wanted Venom, they should have introduced the symbiote and Eddie Brock in Spider-Man 3 to allow character development. Spider-Man could acquire the black costume, and show all of the benefits that come with the symbiote; the final swing could display his speed and strength as he swings before the end credits. Then by Spider-Man 4 they can start introducing the negative aspects of the symbiote, and introduce Venom after an hour or so, letting him become the main villain. Heck, Doc Ock didn't really become the villain until about an hour (the bank fight scene).