It depends on the character, IMO. If you're going to have a long-term major villain present throughout the series, having them simply being "evil" for the heck of it without any fleshed out motivations or backstory just makes it uninteresting.
If a character's just fodder for Spidey to punch around, that's one thing--you don't necessarily need their life story. But when you're dealing with major characters like Green Goblin or Doctor Octopus, it doesn't hurt to expand their character.