I agree that getting hit by back spray seems exceedingly unlikely when on a bike unless you are spraying a dog that is in front of you, which seems pretty unlikely to me.

When a partial can was given to me I tried it and can say that yes it works pretty well.

On the other hand, you can't fly with the stuff and I personally wouldn't bother to track down a source while on tour. Maybe in 3rd world countries even more backward than Kentucky I'd change my tune, but I don't consider it mandatory in what I suspect is the worst area the US has to offer in this regard (MO and KY).

That said, I think that it does good in that it educates the dogs that chasing bikes might not be as much fun as they thought.