Three comments: it is not as easy to hit a dog with the spray as you might think, they are aware you are trying to do something. So it is frequently only a little more obnoxious than a water bottle because they veer away. It is easy to lose control trying to spray a dog especially if there is more than one and they are a bit aggressive and you are paying more attention to the dog than the road. Finally if you are riding a little ways behind a rider who sprays around you might ride through the cloud of spray, not a pleasant experience, not as bad as the dog might have but burning face and eyes can be experienced by following riders. I've had this happen to me.
Steve