Sometimes it's hard not to get carried away by the offtopic posts in a thread... I think most of us have done that at one time or another...
Okay so back on topic...
Why a vent in a teardrop?
Simply, to keep you cooler!!!
Living in a small box in the middle of summer, it can get warm in there. A good roof vent can lower the temperature quite a bit, especially if it's not especially humid outside.
My two favorites are the fantastic fan, which I had in my first two teardrops, and the Northern Breeze, which I now have in my Lil Diner, and my Tab. I like the Northern Breeze because instead of a switch, it has a slide. So I can make it go really slow, meaning really quiet!
Just my $.02 worth!
Mike...

The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...