Most of the time when I decide to use the DF I am pretty much forced to run it as fast as possible because people cannot be slow otherwise they will die (or at least the way they react makes it seem that way). For a lot of tanks it's probably habit, fear of being the target for abuse or simply the bad tanks who are left because the good tanks prefer not to use duty finder. Being a tank really isn't an easy job simply because you take a lot more hate from the community all in the name of getting content done asap.