After our slideshow, Hobbit Houses We Have Known: A Tour of Underground and Earth Sheltered Houses, a number of commenters pointed out that we had missed a whole lot of houses. It's true; we were primarily showing projects that, as the title said, we have known. However it inspired me to go out and find more.
Probably the real reason Tolkien put the hobbits in earth sheltered houses is that he knew them from Ireland and Iceland, two countries where people did it because it was much warmer in the winter. It so happens that it is also much cooler in the summer.