I had to make a thread about this. Ever since BBv1.0, auto brightness has been running without care for battery life.
Who else uses auto brightness?
The only reason that I've been running auto-brightness is because sometimes on BB I get blackouts on my phone. What I mean is that it's brightness goes to the lowest possible and it is impossible to see anything without a bright light on the phone to help me navigate it. I always did a nandroid restore to fix this but it just kept reoccurring. So I ended up turning on auto-brightness to see if that'll fix it for good.
About the battery life on auto-brightness, I don't really see a difference in battery life whether it be on or off.