Well all of said above is truth.... (both of them)
Though I have used them only for very specific thing...
As I used to play Necromancer then I always get 1 or 2 castellans to protect him... an by doing such they stay alive till the end and so they are more dealdier than at the begginnig of the game
I magine a fairly beaten up Aragorn let´s say abut 1 woudn left and 1 fate?
Now this is a job that Castellans are up to...
Combaind with NAzgyl or Necromancer then they make a very quick slice on herose
(better when herose are beaten up but good enough even with strong herose)
Don´t waste them on troops
Too preciose killing machines