Originally Posted by
Teriz
Classes and professions are not the same thing, that's why they can explore similar themes because they are different by nature.
Keep in mind that even if it seems that classes have similar specs, the overarching class theme makes them different. For example, while Fire Mages and Destruction Warlocks may seem similar, Warlocks combine fire magic with Shadow magic, whereas Mages combine fire magic with ice abilities. That makes the specs different. Again, there are things Warlocks can do that Mages can't do, and vice versa.
There's nothing a Necromancer could do that a DK wouldn't be able to do. They don't have a similar general theme, they have the same theme.