If you've seen Spider-Man 3, you will probably find this interview interesting. I think it explains why the film was a little more crammed than it should've been. But even with all the new characters and villains, I don't think it suffered as much as the Batman franchise from packing in too much: starting with Batman Returns (3 villains), continuing the trend with Batman Forever (2 villains, 1 sidekick) and ultimately wrecking with Batman and Robin (3 villains, 2 sidekicks)
Talking (Spider) Sense with 'Spider-Man 3' Director Sam Raimi

Primarily, it was a story that featured the Sandman. It was really about Peter, Mary Jane, Harry and that new character. When we were done, Avi Arad, my partner and president of Marvel at the time, came to me and said "Sam, you're not paying attention to the fans enough. You need to think about them. You've made two movies now with your favorite villains and now you're about to make another one with your favorite villains. The fans love Venom. He is the fan-favorite. All Spider-Man readers love Venom. Even though you came from '70s Spider-Man, this is what the kids are thinking about. Please incorporate Venom. Listen to the fans now."

As much as I like the story of the black suit and Venom, I don't think he should've altered things based on Arad's perception of fan desires.