It is a mixed feeling for them. Some people like that they sift sand but if they are large enough they can kill snails, crabs, and small fish.
I disagree. A bristle worms diet does not change as it gets older. Bristle worms also do not have any way to attack and kill a healthy fish, no matter how small or slow it is. Bristle worms do not eat snails or crabs, unless of course, they're dead. It's only a myth that they eat fish, coral and invertibrates.
I have 2 bristle worms around 24" in my tank. They do a great job at eating detritus, left overfishfood, etc.
Graham