Snow pants without vents rely entirely on the fabric's breathability to manage heat, which works fine if you're mostly sitting still or riding in bitter cold. If you're just doing casual resort laps, taking it easy, or wearing fleece-lined pants for warmth in frigid conditions, you probably won't miss vents much. They're also common on budget-friendly pants to keep costs down.
The problem shows up fast if you're an active rider. Hiking to a line, skiing moguls, or just working hard on a warm spring day will leave you sweating with no way to dump that heat quickly. Once you're damp, you get cold the second you stop moving—a miserable cycle. Even highly breathable fabric can't keep up with heavy output.
If you ride actively or in variable conditions, skip ventless pants. At minimum, look for inner thigh vents. Save the no-vent option for cold, mellow days or après-situations where overheating isn't really a concern.