The underlying assumption of brainstorming is that if people are scared of saying the wrong thing, they’ll end up saying nothing at all. The appeal of this idea is obvious: it’s always nice to be saturated in positive feedback. Typically, participants leave a brainstorming session proud of their contribution. The whiteboard has been filled with free associations. Brainstorming seems like an ideal technique, a feel-good way to boost productivity. But there is a problem with brainstorming. It doesn’t work.
And, from the same article, this:
According to Nemeth, dissent stimulates new ideas because it encourages us to engage more fully with the work of others and to reassess our viewpoints. “There’s this Pollyannaish notion that the most important thing to do when working together is stay positive and get along, to not hurt anyone’s feelings,” she says. “Well, that’s just wrong. Maybe debate is going to be less pleasant, but it will always be more productive. True creativity requires some trade-offs.”
There’s a lot more in there, all of which is well worth a read