Firstly, I still don't buy the whole 1000 years thing. Turalyon certainly hasn't been out there for 1000 years. Alleria may have been trapped in some weird place for 1000 years separate form Turalyon, but otherwise it's hyperbole.
On topic: I don't think Alleria was ever a holy person. She was just a ranger or huntress. She's not found a powerful magic she can use against her enemy. Why not use it? She doesn't have a religion-esque belief like Turalyon does to prejudice her against using the Void. I see it as totally logical form her point of view. Also, as for the story it helps their direction a lot. It helps show how the Light is not so perfect as presented earlier in WoW, also shown in the Illidan cinematic with X'era. It's presenting the Void and Light not as good and bad, but just two different forces.