This is just my 2 cents, and it's what a vet/chiro told me when I was running (a long time ago). Polos offer minimal support (if wrapped correctly), there's to much give in the material. They are best suited for protection against abrasions. If you are doing light work (i.e. walking, some trotting, no serious turning or stopping, etc.) Polos are a good idea. They offer just enough support, but not to much.
If any "hard" work is involved boots are a good idea MOST of the time. While nothing is going to give 100% protection/support to a 1200lb animal, boots are designed / made to have less give and more structure than polos.
BUT, she gave me a serious warning. To much of either can be a bad thing, and she explained it this way. If you wear a knee brace for support, unless you excersise the knee WITHOUT the brace, it will never be strong enough to be used without it. Your body adapts to it being there, and uses other muscles accordingly. It's the same with boots / polos. If used EVERY SINGLE TIME, you are losing the ability to strengthen muscles without support. So the second you don't have them on for whatever reason, you run a greater risk of causing an injury.
I actually did this as an experiment for high school science class (it was a semester long project). I had a knee injury back then. So I went 2 weeks wearing an ace bandage all the time. Then 2 weeks with nothing ever. Then 2 weeks with using it just with strenuos activity. Then repeated the same using a pro-choice knee brace. How I determined which was working best, I just kept track of a daily pain scale (1 being no pain - 10 being the worst) and averaged the results. This is what I found. With the ace all the time, it helped but I was still in some pain (a 6). No ace, sort of the same, but a little more pain (a 7). With activity a 6. With the brace on all the time was a 2 or a 3, But no brace was up to 8. Only with activity was around a 5, but towards the end the pain kept getting less and less. So what I learned was that an ace helped, but not a lot. A brace helped A LOT until I took it off, but when used only when needed I got better faster.
And from my experience she's right in regards to horses. The horses that I see that are ALWAYS worked in boots or polos without an exception are the often the ones that most often come in from the pasture limping. The ones that wear them only in certain footing or conditions are the ones that seem to hold up much better than the others.
So that's just what I've noticed. It's not right or wrong, either way