Yes and No! Yes if you got a sand bed and no Clams and they will scraf up left over food and what not that accumulates on the bottom. No if you got clams as they can sometimes annoy the clam and cause it to not thrive. At least thats been my experiences with them.
I know bristles can be good, and most are. With how red these are, (yes most of those pics are the same one), I was concerned of them being fireworms. I saw a couple larger ones hiding in the bigger rock in the back, and I was under the impression only fireworms get big, but I honestly don't know how big those are. I just saw them for the first time a couple nights ago, but obviously they have been in there for a while.
I ask because I am getting ready to set our 65 back up, and move stuff from this 29 biocube to it. Trying to figure out if I need to limit what I move.
Good for the tank, bad for the fingers. Make sure you look before you grab or you'll have some discomfort for a few days. It's not bad, but it's bothersome enough that you won't want to do it again. The closest thing I've found is getting fiberglass spliters in your fingers. Not from insulation, but from splintered pieces.
I've always had a robust bristleworm population and have kept several clams over the years. Worms were never responsible for their downfall, but they were quick to start eating them when they died.
You'll have more of them if you feed a lot, fewer if you feed less. The population will self-regulate.
Most people consider them beneficial, some even buy them. In addition to possibility of getting hurt, do not leave a tank with them fish-less for to long ... I once did, and they became so big that they were swallowing my baby Banggais ;(