That is a question countless have posed and something intuitively tells us they can't be good.
In my mind, the short answer has been, the small ones are fine, but the big ones might become a problem. They actually can be viewed as beneficial members of the food chain.
Now I believe all bristleworms should be eradicated. I recently had an experience with small bristleworms causing an Acan infection. They either fed off the coral or constantly irritated the tissue with their bristles until it became infected. Then the infection spread to other acans. If left unchecked, this could have led to the deadly "domino effect."