I believe that a horse can become dependent on boots to stay sound, so I only put the SMBs on when I'm going to be doing something very strenuous with Cody, or we're working on something new where there's a very good chance he could interfere with himself. I only use the boots 50% of the time at most, and that time is decreasing the older Cody gets (he's three now).
There are pros and cons. The obvious pro is that the boots provide protection. The cons are that your horse needs to learn to place his feet carefully and correctly, and he won't learn that if he never knocks himself. Also, boots don't breath well, and they can cause heat buildup in the leg which can become uncomfortable over an extended period of time. And if dirt or sand gets in between the boot and leg it can be abrasive and uncomfortable. Also, the horses lower leg will not become as resilient to tendon and ligament injuries if boots are used all the time.
Don't get me wrong, I love my boots, but I only use them when I think it's necessary.
And I agree with the above posts. Polos don't offer any significant support to tendons and ligaments, but boots do (if they fit properly).