Thirded, depending on the circumstances.Agreed. When I run 3.x, there is a class from a third party publisher which gets its powers from bargains with infernal forces. I have that limited to NPCs. The same goes for certain PrCs.
If I were running 4e, I would place the same limitation on Star Pact and Infernal Pact Warlocks.
E.g. "For my next campaign, I'm thinking of a set up where the PCs are the last remaining loyal followers of a LG church, fighting against the previous leadership, who have all fallen to the dark side and become blackguards."
"Cool. Can I play a blackguard?"
"Er, no."
On the other hand, assuming a standard D&D campaign :-
"Can I play a blackguard?"
"No, I've banned them because I think they are overpowered. However, most NPCs you fight will have levels of blackguard since I think that makes them tougher than their challenge rating and I won't be awarding any extra XP as a result."
Is not cool.