brisstle worms are just something that will be in your tank if you have live rock and sand, don't think you really have an option there wether to keep them or not, they will just come back. as far as being good or bad, I have seen were people try and remove some, and have heard that getting to many is a bad thing, not sure what's bad about them, other then you don't want to grab one and get stung. I have always had them, never tried to remove any except for a few fire/brisstles that I found that were pretty good size, but have never had a problem of any kind by having them.