i would have told you they were useless a week ago... however in the past week i woke up in the morning to a hard drive that i had begun defragging over night and it was hot as heck and making crappy sounds. needless to say it was dead, of course this probably has more to do with faulty hardware, but i guess it might not have died if it didnt get so hot. so i guess they might save you from an unexpected cost later but in normal situations they probably arent needed.
piece of advise: stear clear of ibm deskstar/deathstar.