From what I understand, venting isn't as critical as you'd think in the desert. Yes, you need something that breaths a little. But to keep cool, you don't want that 110 degree wind blowing on you. Instead, I hear most adventurers wrap a wet towel around their neck.
To further expand on this train of thought:
When it's really hot, your sweat evaporates super fast. This creates a situation where you can dehydrate or get your electrolytes off balance in a hurry because your body is constantly sweating out your water trying to cool you. Also, IMO, your sweat/wet t-shirt/cool towel dries out so quickly that it becomes a hassle to keep it wet/drink enough water.
If you put a wet t-shirt on and ride with a mesh jacket on, you will get GREAT cooling, for a few minutes. You might even get cold. But under a jacket that allows air movement with vents, you'll get good, or maybe only adequate cooling for a much longer time. This is important...
As your body gets cold- especially your extremities, the vessels begin to constrict. This is a natural response to preserve heat when it's cold out. By putting a wet t-shirt or towel around your neck, then allowing the water to evaporate out of it at a high rate, you're cooling your body to a point where it may begin to shunt the blood to your core. This is counter-intuitive to your goal. You're making a section of your body cold, while not cooling your core.
Taking the same wet shirt/towel, and controlling the rate at which the water evaporates, you don't lower the temperature of your skin to the point where the vessels constrict. This keeps the blood flow going, and the cooling effect reaches your core and your brain. For short rides it probably doesn't make a difference, but if you're going to be in the desert for a day or longer, you will be better off with this approach.
Think of it like this:
Would you rather have
cold for 10 minutes, or
cool for 30?
There's also heat transfer occuring from the air to you. A 110 degree wind WILL warm your body. The more wind, the more efficiently it will warm you. Keeping the wind off helps. Lowing the temperature inside your jacket is better. Effectively managing the air flow/heat transfer/evaporation that is occurring around your body is the best. Not riding in the extreme heat is ideal.
