Really? They've had a couple of hits--"That's What You Get" and "Decode" were pretty huge in 2008 (the latter was in Twilight, so the Twihards became Paramore and Muse fans overnight).
I didn't even know "Only Exception" was a hit until she sang it on MTV's Awards thingy.
Personally I think their songs "Brick by Brick" and "Ignorance" are pretty good, but most missed them. $20 says though that "That's What You Get" ends up on a future episode of Glee...probably this season.
P.S. To all the haters, I like "Airplanes."

I'd prefer to hear that than Britney or the undoubted soon-to-come Katy Perry episodes.
P.P.S. I know Rachel is singing "Only Exception," but I'm holding out hope it is somehow about Brittany and Santana while Rachel is just belting away in her auditorium. One can hope.