Dirt helmets are all about being lightweight and breathable. No one wants a 10 tonne insulated brick on their head when hitting the jumps.
For a full face, this is as breezy as you are going to get when out there on the roads.
The con to the dirt helmet is the wind noise that comes with the open an airy design. For the dual-sport 60/kph riding that most us do in Vietnam, this isn't a big deal and ear plugs can resolve the problem.
The difference between a cheap dirt helmet and an expensive dirt helmet you ask? Safety comes into play, but it is more about weight saving and the balance on your head. Expensive helmets balance perfectly and feel weightless, whereas cheap helmets pull your head down.