Imagine now for a moment that you have a zerg-like creature that is heavily armored, it can fly and it has some scary looking claws.
It flies towards a Battlecruiser. Locks its claws to it and starts tearing it apart. Only his allies can shoot at it and the Battlecruiser can't shake it off.
It might not be as fast as a phoenix but if the phoenix isn't careful and gets too close it locks his claws to the phoenix and the phoenix can't shake it off on his own.
You better micro your vikings like a mad man to pick off as many of those rippers before they close the distance and start tearing the vikings apart. Some pilots might even make the decision to land the viking to get rid of that threat above.