Some of the big stars ARE using Auto-Tune on shows, but ONLY with an engineer to run it, and it never gets into the monitors. If someone 'rides' the controls, inputs key and mode changes in (or a sequence track puts them in at the right point), and changes the amount needed and the speed of the re-tune in real time, then it does work (to a certain extent).

But unattended, it's a recipe for disaster!