While I agree that non-Hermodice species of bristleworms are generally beneficial and do not advocate removal of them in most situations, I also feel the need to point out that they are not 100% safe in all tanks. The two conflicting views on these weorms that exist in the hobby, that they are completely safe and that they are pure evil, are really too strict in either direction. The reality I have seen is somewhere in the middle, although much closer to the view that they are completely safe than to the alternative.
Based on what I have seen of the animals in my own tanks, these worms can go undetected for possibly years, so I am quite convinced that they can and do exist peacefully and harmlessly in most tanks when the ecosystem is properly set up. However, a sizable population of large non-Hermodice worms (which are probably Eurythoe) that are peaceful for years with sufficient access to food can suddenly turn quite destructive if deprived of food and not controlled by predation before they start to starve. Starved individuals will swarm and rasp at things that they wouldn't normally, including algae, corals, snails, and small, molting Crustaceans. The spines carry irritants, and snails can get stuck to death when the worms enter the mantle cavity of the shell, poke around, and trigger the snails to close up on the worms. This doesn't damage the worm's soft body, but forces many spines into the snail's soft tissue. The snail will then open up again quite quickly after being stuck, the worm forces in more, and the process repeats until the snail looks like a pincushion inside and slowly stops moving. Corals and freshly-molted Crustaceans are vulnerable because of softness and immobility, where they can either be rasped to death or rasped to the point where they don't recover for other reasons.
However, the situations I just described are incredibly rare and I would assume should not occur in the average well-kept tank. Still, it's something I've seen firsthand with worms that are clearly not in the infamous Hermodice genus. Non-Hermodice worms may be 99% safe, but one should be watchful for any large worms that start to be unusually active during the day, particularly if they ignore prodding with tongs.
EDIT:
What is the likelyhood that these bit our clown fish the other week?
What do you mean by "bit"? Fish are likely to get stock with bristles, which can give them a furry patch for a while but will not harm them as long as it doesn't become infected. A healthy fish will not hold still long enough though for just about any fireworm-type bristleworm to get a hold of it with its mouth. Exceptions would be worms that are technically "bristleworms" taxonomically but aren't what the average hobbyists thinks of when they use the term, such as the Eunicids (which have substantial, pointed jaws).
