if you are scared of them, yes they are bad. Some people are wuss about them, "OHH THEY ARE SOO GROSS LOOKING, THEY REALLY CREEP ME OUT! I DONT WANT WORMS IN MY TANK!"
I say, they eat alot of the food that your fish dont eat, they eat the poo from fish, and they keep your tank pretty clean. Call me crazy, but I dont need un eaten food sitting in the bottom of my tank and I dont need fish poo in there as well.