My best friends can. Bolin is an Earthbender and Mako is a Firebender. Korra is extra special, she can manipulate all four. Only one person every generation can, and they're supposed to keep the world in balance.
So I guess I'm the token normal friend. My... father hates benders because a firebender killed my mother in a robbery. But I can't blame others for something one person did, you know?
And they need me because I'm the mechanical genius. ;)