A vented pocket is a clever dual-function feature—the pocket unzips to double as a ventilation opening, letting you dump heat on warm days or during high-output riding. It's essentially two features in one, which keeps the design streamlined and saves a bit of weight.
The trade-off is that you can't really use the pocket for storage when you're using it as a vent. If you unzip for airflow mid-run, whatever's in that pocket is either falling out or getting exposed to the elements. It's a niche feature that makes the most sense for riders who prioritize a clean, minimalist setup and typically run hot enough to need ventilation.
For most resort skiers and riders, separate thigh vents plus standard pockets offer more flexibility. But if you like the idea of fewer zippers and a simpler design, vented pockets can be a nice touch.