http://www.aad.org/media/background/news/Releases/Hair_Care_on_a_Budget/

This is an article I found on another website.  Interesting.  It's definitely aimed at people who blowdry and color regularly, but it has some interesting stuff in it, particularly the part about the BBBs!!

Read through the article. Pretty interesting. I agree with most that was said, but I don't really agree about the BBBs.

Counter arguments:
BBBs can be damaging if you're trying to detangle with them. Brushes, whether BBB or wood or plastic, should only be used on pre-detangled hair.

BBBs should be made of natural boar bristles, or hair. If you are brushing along your hair with hair, this should not cause any damage. Otherwise, your hair rubbing on itself could cause damage. The same concept goes for washing clothes. If you rub coarse cotton against silk, the silk will abrade. If you rub silk against silk, you are much less likely to damage any of the fibers. That's why you wash delicates separately from your regular clothes.

The "natural grain" of the hair? If they mean cuticles, that implies the grain goes from scalp to tips. If you are stroking UP to your scalp, that would definitely be bad! Stroking down, however, like stropping a razor, should help make the hair cuticle lie flat.

EDIT:
AHAH! After reading her phrasing a couple of times, I think I've figured out what she means. When a BBB is made, the boar bristle is inserted into the brush TIP first. The white tips on the bristles you see are the roots, the part that was attached to the boar's skin. That would mean that the cuticles of the BB's themselves point down towards the bed of the brush. Brushing your length with the bristles would be very much like "backcombing" your brush's bristles constantly.  I guess I can see where this might be bad, but I do not know enough about the hair structure of boar bristles to really make an objective statement. At a guess, I still would think that this isn't too damaging to hair. You are still stroking down your length. The potential up-raising of cuticles on the brush might be minimal since once again, it is hair against hair. Having the open-pinecone effect going down your hair might be better at distributing oils, removing dirt, and closing your own cuticles, than the reverse.

If anything, I'd use the above more as an argument to  why you should regularly be replacing your BBB at intervals instead of not using one at all.

I agree that doing any of this too roughly might damage your hair, but if you're always being gentle to your hair, and detangling first, I can't see how brushing your hair with a BBB could be very damaging.