Apparently Sam Raimi didn't originally want to add Venom and Gwen Stacy to Spider-Man 3, it was actually Avi Arad that turned him around and persuaded him to do it for the fans.

Raimi was focused on the idea of having just Sandman and Spidey battle it out, but then Arad turned him around when he started talking about the fans and what they would want to see in their film.

At a news conference Raimi said:

"It was really about Peter, Mary Jane, Harry and that new character...

...Avi Arad, my partner and the former president of Marvel at the time, said to me, 'Sam, ... you're not paying attention to the fans enough,'" Raimi said. "'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, and even though you came from '70s Spider-Man, this is what the kids are thinking about. Please incorporate Venom. Listen to the fans now.' And so that's really where I realized, 'OK, maybe I don't have the whole Spider-Man universe in my head. I need to learn a little bit more about Spider-Man and maybe incorporate this villain to make some of the real die-hard fans of Spider-Man finally happy.'""

The quote comes from SciFi Wire who also have quite an extensive quote from Sam Raimi about the inclusion of the character of Gwen Stacy, which wasn't in his original plan either.

This is interesting to hear as it kind of offsets the idea that Arad the Producer is the one who is the big baddie. Sure he wants to make it more appealing to all the fans because that increases ticket sales, but at the same time he's giving the fans what he wants.

You might have thought that this was the other way round. Still, Spidey and Sandman would have been just as fine if we then saw Venom in the next film. However there's not a lot of complaint of having the two of them in the film.