That's interesting considering it can really only swat away insects around it's behind and is useless for the rest of the body.
I've noticed that when a fly lands on a horse's front area, the horse will twitch its muscles in that area (sort of like if a fly lands on your are and you jerk your arm to get it off), but in the back area it will swish it's tail. Maybe it can't twitch its muscles in the back area like it can in the front area.
According to the article, the goal is not to hit any insect with the tail but rather to create an air wave that is impossible for insects to navigate.
Based on my of observations of cows while working around them another (or IMO rather, the) primary goal seemed to be to swipe or scare away flies and horse-flies that have already landed.
It's not uncommon to see 2 horses standing next to each other, but facing opposite directions, so that each horse's tail can flick flies away from the other horse's face.
There are some nice soft parts there that aren't protected by the animal's skin
The behind tends to be the smelly end on horses though.
Don't forget they're herd animals. They also protect each other (using their tails) when insects are bad.