The skeletons spawn in the middle; if you tank the boss AWAY from middle then even with a fail group that kills skeletons before tanks can move them you still have a chance of killing them before they reach boss to AOE. THAT is why tanking the dragon away from the middle is actually a better strategy, it's better for dealing with fail DPS.

It also has the added benefit of giving you twice as much time to kill the skeletons if they are successfully moved to the ends of the platforms as they have to cross the entire arena rather than just half of it. Considering the quality of players I see in the DF sometimes you may want that extra time...