I've never been a fan of closing certain classes for some alignment, especially since it didn't always make sense and was terribly restrictive. Why can a paladin only be lawful good? Why couldn't any evil or chaotic god have his own paladins? It always seemed extremely stupid to me.
Especially in the older DnD games, this often led to some rather strange combinations. Probably the most extreme example will be Nathyrra from HotU who was assigned lawful evil due to gameplay, despite the fact that the whole character was more chaotic / neutral good.
The same problem could be found in the new pathfinder. Seelah is probably the most visible example, all her behavior is neutral good, yet she somehow manages to maintain her powers which is impossible for the player character.
