I see.
I have made the roof flat, so entire
project took me no more than 2 hours for connecting the pallets and 2 hours for grading my stone hard soil with hoe and a chisel. I did not worry about any roofing material, because for 6 months of the year I get at most 0.1" of rain. When it started raining I covered it temporarily with black tarp and pressed with another pallet.
If I had to do a sloping roof or put some roofing material I would consider it not efficient any more - I would have to put my real labor and use temporary materials and I prefer to use my time to build something real that will last few hundred years. So in my opinion it makes sense to use pallets for ad hoc, quick structures that will be erected really quickly and this way - cheaply. Also, they do not look really ugly like some hybrid of abandoned random materials. If someone wanted they could be brushed or charred rather quickly and then oiled.