Full side zip vents run the entire length of the leg, giving you maximum airflow and the ability to take your pants on and off without removing your boots. They're incredibly convenient for backcountry touring, where you're constantly switching between sweating on the uphill and freezing on the downhill. The rapid heat dump is essential for managing those extreme temperature swings.
For most resort riders, standard inner thigh vents are perfectly adequate. Full side zips add weight and cost, and you likely won't use their full potential if you're just riding lifts. However, if you run hot, ride in variable spring conditions, or just love the convenience of boot-friendly changes, they're worth considering even outside the backcountry.
Think of it this way: if you're touring, they're basically mandatory. If you're not, they're a nice luxury but not a necessity.