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When and how would you use them? In what kind of army list would you use them? Just want to know because I have all 6 but I havent used them in a current list before.
I'd consider using them in Apocalypse games, as for the most part I don't like sinking so many points into a single model. I will say though, for a standard game the only ones I think would be worth it would be Karandras or Jain Zar. Really, the addition of either one of those models doesn't change the strategy involved for each squad that much, just gives them extra hitting power. Granted, I've never fielded any of them in a standard game so that's all based on conjecture.
I think you need to spend more time on your floor.
2500 Black Templars
I field the Great Court of the Young King in Apoc games. I used to add the PL's into their squads but they never seemed to have enough punch to justify their points. For the same points, I now add more Skimmers and/or a Farseer, perhaps even The Farseer. This seems to be more effective. I have all the models because they look awesome, especially with a squad of their aspect.
Even in higher point games I still wouldnt use them. For their points you can usually get a whole extra squad of their aspect which would have just as much punch of the PL alone ( minus a special weapon or power ). Plus, I would rather take the more flexible Farseers and Autarchs or even Avatar or Eldrad.
the only time you actually should field a PL is when your playing larger games or even apoc. in smaller games it's totally up to you if you want to lay down about 200 pts for some extra hitting power that will be sorted out prett quickly if yuo don't watch your steps