Never

Zest-O rootbeer tastes like sarsaparillac mouthwash. Only drink it if you intend on rinsing and spitting. If I were to mix normal rootbeer and listerine, out would pop Zest-O rootbeer. I would rather drink water than Zest-O rootbeer, which is a rather significant point - because I'm stupid enough to dislike drinking water. If I could choose to wash my car (my hypothetical car) with battery acid or Zest-O, I'd pick battery acid hands down. If I had to poison my arch nemesis with Zest-O rootbeer or absinthe, I'd choose Zest-O - purely for the fact that they'd suffer even more.

If I were in a desert, with the choice of drinking Zest-O rootbeer or lie there dying of thirst? I would drink the rootbeer, silly.