Originally Posted by Mjölnir
He does advance the plot in that he didn't only go crazy, he got some insight into a cosmic phenomenon (the one being exploited by the villain). He's providing crucial information to stop Malekith. That's why I'd sooner cut Darcy than Selvig as she is just the funny sidekick.
I admit I'm biased with regards to Darcy, aside from the fact I find Kat Dennings attractive which we've gone over, I don't like Selvig. I'm an astrophysicist for a living, and characters like Erik Selvig, Steve Urkel, etc bother me when I see them on my screen. It's not how people I know and work with behave and think, it's an offensive and insulting stereotype. I don't like the scientist stock character, and it also bothered me in Pacific Rim and in Agents of Shield.