The simple answer is that many people believe Bittorrent and other fragment-download programs (Emule, etc), when used for DAYS or WEEKS consecutively, can put a large amount of strain on your hard drive and damage it.
Usually if your drive fails, it's for a reason completely unrelated to bittorrent; it's my belief that these claims are mostly bogus.
People kick, knock, hit and abuse their computers constantly, introducing dust and vibrations into the harddrive, where it of course fails after a single freekin' year...and they blame Bittorrent? I don't think so.