exactly what tim/steve say (says?)
and as the swarms simply are additional LP for the Lord and cheaper I had more success with them then with 3 wraiths
they can unleash heck of a fight but normaly they die by being shot to death for me ... and my opponents have their long range fire support no matter what I play be it Necrons or Orks.
Maybe I play it wrong but they die to quickly here for my taste.
Good unit ... but not unbeatable