I use polos every time I school lateral movements (a LOT of the time, then). They offer cushioning if a horse hits himself making poor movements.
I use boots when jumping. However, not all of my horses jump in boots. Some jump bare legged by THEIR preference.
As for PVC, it is very important that you use the correct grade of pole. PVC comes in two thicknesses. ALWAYS use the thicker, even though it is quite expensive. The thinner grade shatters into razor sharp edges if a horse steps on it and it breaks. I have seen bad arterial bleeding from a horse cut by one. They are not meant to be exposed to the sun and will get very brittle much quicker than you would expect. Always store PVC out of the sun or be prepared to replace them every year.