I found this website which had an interesting bit on polos and their uses/whether they really provide any support... MyHorse.com - The ABCs Of Using Polo Wraps Uses
Aside from an eye-catching appearance, polos offer some protection to the lower leg from nicks, hits and rubs. Horses that tend to “run down,” meaning their fetlock contacts the ground and can be scraped when galloping may get sufficient protection from polo wraps. As for actual leg support, you’ll find a wide range of opinions from polos being terrific support to their being so soft and stretchy that they provide no support. What they can do, though, is help hold the tendons and ligaments in good alignment so that they’re protected to some extent from sudden shifts in position and overstretching. If you’ve ever had a sore tendon or joint and used an elastic wrap you have some idea of how they can indeed improve comfort. In addition, when the horse has a sore/wound/abrasion, polos may be a better choice, at least until the area heals, as they’re less irritating than most boots on a sore.
So it seems that may offer some support, according to the article above, although not to the extent that sport boots do obviously.