Smaller bristle worms are marvelous scavengers, delving deep into crevices to feed on trapped organics. Really big ones might cause issues - some inside the tank, and some outside, when your spouse reacts to that giant, horrible, disgusting ... THING ... winding its way through your aqauascape . . .
Keep in mind that if you've got them in your tank, there's always a chance you'll put your finger on one while you're messing about in there. The bristles are like fiberglass - they go into your skin, and don't remove easily. Not terribly _dangerous_, just _really_ annoying.
~Bruce