The reason this line having recent success is a good thing is not necessarily because of anything inherent within the line, or about it, but because of the implications it has on the rest of the lineup.
This provides balance. It allows Stepan to not be forced to center Callahan. It allows three different lines with a potential 30 goal threat on them. And, it allows dependable 4th line players like Boyle, Halpern, Asham to not be forced to play above their heads.
Certainly Pyatt and Hagelin are playing "above themselves" in this current assortment, but the important thing to me is good balance and a group of lines that function together at even strength. Cally won't be at a lack of ice time because of PP and PK minutes.
It may be a good line But any success it has just makes the rest of the team that much better.