i have never had a problem with them. I think that it is worse for sps tanks than a mixed. hook your tank up with some crabs and they will be kept to a minimum
well.. they can be bad if gotten out of hand... you can get them from anywhere... rocks, frags, cheato....best thing to do is get a fish that eats them... using chemicals to kill them is not good practice as there is a great chance of tank wipe.
Yup. To further explain Fred, flatworm exit causes them to die, once they die, they release a toxin, if to many die at once then you have a tank crash from the build up of the toxin. At least that's the explanation I remember from browsing the web.