I wouldn't even use the polos in working. I remember reading too many stories of them coming undone and the horse coming undone, often the rider too. Imagine stepping on a flopping piece of bandage of one leg with the following leg at good speed. Snap! One lady was paralyzed I believe.
I saw one lady (girl really) jump a whole grand prix with an unraveling polo wrap. It was on ESPN or Outdoor Channel I think (MANY moons ago). Amazingly they made it all the way around but at least he wasn't traveling at great speed if they hadn't.
In the mornings most of us put electric tape over the polo after we are finished wrapping. While I would never work one in polos, I'm one of the group that thinks it's silly not to wrap for galloping and backtracking. Such a simple step could prevent so much. I've seen a horse with grabs on overreach and totally severe the tendon on a front leg. Not a risk I'm willing to take.
Polos have their place, on the racetrack they are used for galloping in dry conditions. Only an idiot would use them for breezing or on a sloppy track. I've seen just about every kind of bandage come off, except vet wrap.
I've watched Saratogas, ace bandages and trace bandages come unraveled from underneath, even when seemingly put on correctly. I remember having an arguement with an assistant about taping a polo, because it was brand new with good velcro he thought it wasn't needed. Yep, came undone halfway around the track.
However, they have been used under vetwrap on a bad track in the morning only. Just some added cushion and you don't have to use an entire roll per leg. One of the top trainers in the country worked horses with this setup.