You wouldn't necessarily use mechanical doping to win the general classification, or even a particular stage.

More likely, you'd use it on select stages for very specific reasons... for example, a rider could use it to avoid the time cut on an ITT stage (effectively getting extra rest vs their competitors). Similarly, a pure sprinter could use it to stay in contention on a punchy "sprint" stage (like a stage that MvdP might be a favorite instead of a pure sprinter).

Edit - I don't think anybody is doing this at the top levels of pro cycling. Maybe in regional racing (masters, etc).