[Sk8r]

1. they have no jaws and cannot bite: they suck liquid, decaying waste and will 'clean' dying coral and dead fish in that way. In that sense, they can save your tank. They are extremely valuable cleaners who can get into places nothing else can reach. Every tank should have a handful. My 54g has supported about 50, plus 4 ten-incher oenones (so I am told they were). I have a very healthy clam, huge lps, sps, fish, you name it. They bother nothing. The ONLY precaution I take is to putty up the cut end of any lps coral I frag to deny them entry there, because they're always looking for homes, and corals just need their 'feet' protected from hassle.
2. yes, sometimes they give a fish a few bristles. These fall off after 2-3 days and the fish learns not to shove a bristleworm.
3. their bristles find the grooves on our fingers and stick in hard. This hurts. use latex gloves when handling your rock: do not accept repeated bristleworm stings as ok: you can get sensitized to them, and this is not good.
4. a tank without bristleworms is missing an essential member of its cleanup crew. If you want a resilent tank, have them; and if you must have a fish that eats them (wrasses, etc) establish a fuge where they can live and breed along with the copepods, etc, and they will help clean up whatever detritus gets to the fuge. A few will get loose and turn up in the tank, maybe taking up residence in the rocks where the fish can't get at them. At least you will have a breeding population of them.