I don't like the line, but I do think it has stayed because of defense. That is, Babs might not trust Tatar to play against Toews, Thornton, Kopitar, etc. I think Dats and Abby/Cleary could makeup for any of Tatar's deficiencies, but that's the only explanation I can think of. Flip should return and the line will be split.
The 2nd line also has better D pairings with them so I don't think it's Tatar's defense that's any kind of issue. Plus, Abby is no defensive dynamo either and doesn't even play on the PK. Tatar skates super fast which can usually make up for defensive mistakes anyway. I simply think it's the stupid "seniority" rule. Tatar is newer and therefore cannot move up ahead of certain "other players" no matter how good he is. Even when it is painfully obvious he should be.