the thing is, they can play incredibly offensively with lugia and hippowdon (saccing them immediately), and they won't need to worry about us sweeping them with rayquaza or groudon because scarf shaymin-s can already revenge and sweep our team. (also i assumed darkrai was running modest because its not like it needs timid to outspeed our team)
they lose defensive integrity by running a fast scarfer, but this defensive integrity is unneeded because the offensive power they gain by running a fast scarfer is more beneficial than their defensive integrity.
that's just my opinion though, lol.
Great then we have a 6-3 mon advantage (excadril's been practically neutered). Do I need to expound about how great that is? NP LO Darkrai still doesn't OHKO Ogre, and if we really are pestered that much by the fact they have a faster scarfer we can run something like an Extremekiller or Scarf Mewtwo, and we still don't loose defensive synergy. Scarf Mewtwo still pretty much breaks the whole team and Extremekiller can run Band if it needs to end those threats ASAP.
If we were to take the defensive approach a Support Fighting Arc. can patch up problems vs. Darkrai while Scarf Skymin is still handled by our Zekrom. If they choose Mewtwo Support Dark Arc can patch up problems. Also lol even Support Dark Arc. lives +2 Focus Blast.
Point is even by using that stuff we can still patch up our problems, still leaving the other team at a disadvantage if they do choose to make the offensive turn.