Low breathability hip protectors use dense materials with minimal ventilation, which means they trap heat against your body. This actually makes them a solid choice for very cold weather skiing or racing, where the extra warmth is welcome and the denser construction provides maximum durability.
However, for most skiers, low breathability is a recipe for discomfort. Remember that hip protectors are worn under your ski pants, so heat builds up quickly—especially with insulated layers. On all but the coldest days, you'll likely end up with sweaty, clammy padding that makes you want to take them off mid-run.
If you rarely ski in deep cold or race conditions, moderate or high breathability will serve you much better for all-day comfort and keep you actually wearing the protection you bought.