The wraith's only good thing is to be used for Anti-Air. Outside of the mission you got them I never used them. Even then on the higher difficulties I opt'd to use ghosts instead of wraiths.
Banshees are much better anti ground unit and viking is the better anti-air unit. You pick depending on what you are fighting. Wraith's only saving grace is that it can cloak, but most anti-air missions you can out range whatever your aiming at and be fine using vikings.
Valkyries made wraith pointless against air targets back in BW. Now we got valkyries that can attack ground as well. And for cloaking we got banshees. --- Setebos, Setebos, and Setebos! 'Thinketh, He dwelleth i' the cold o' the moon. http://www.youtube.com/watch?v=zIjVvnO5lgM
Valkyries made wraith pointless against air targets back in BW. Now we got valkyries that can attack ground as well. And for cloaking we got banshees.
Valkyries were made for air splash vs things like mass muta or to kill intercepters faster. Same thing for corsairs except that was a secondary function and their primary was to block anti air ground units/static defense.
They were barely used wraiths had more of a function against anti-air until BC tech was in full swing.
It depends on what do you want. If you're going for harassment, then use Banshees... then again, I doubt you're going for harassment in the campaign. Otherwise, if there is need for some air superiority, Wraith is the best choice. Otherwise, go for Banshees, as they are pretty strong against any ground-to-air that Zerg or Protoss can throw at you.