Take, for example, the American public's acceptance of atomic technology during the Cold War. Then the genie was already out of the bottle, and there was simply no way to return to the pre-nuclear age. In retrospect, atomic weapons may have actually saved lives, because there have been no world wars since the allies incinerated Hiroshima and Nagasaki in 1945, but that's beside the point. All a truth machine can determine is what the subject believes to be true. Such information is often worthless, but occasionally very useful.
Most people are trustworthy most of the time, but few are trustworthy all the time.
We all have lapses, dark impulses. With over five billion people in the world, some of us are bound to be both dangerous and unworthy of trust, and no one alive can distinguish honesty with total assurance. Expert liars are very common. Personally, I tend to trust people to act in what they consider their own long-term best interests, which is about as good as it gets. A truth machine would cause the right people to be trusted, and force all others to become more trustworthy.