Well, there's also the fact that Avada Kedavra is an Unforgivable Curse, and since many good wizards are above those, they tend to not use them. It's a pride/nobility sort of thing.
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Will not remove this line of my signature until the Seahawks, Jazz or Rockies win a title (Started 5-16-09)
I know I'm not getting rid of this signature, BTW.