Those are NOT bristle worms, they are Eunicids unless I'm mistaken... which I would be surprized if I am. They are an omnivorous/scavengers, most of their diet is detritus/left over foods, but the bigger ones in some cases can be a risk to sessile inverts and maybe a few coral spiecies, generally not an issue though. They live in tunnels, and some of them even burrow into the rocks to build their tunnels.
I would keep them unless you find sessile inverts disappearing (tube worms, clams, etc) or corals getting damaged. As like I said detravours, so they will help keep the tank clean.