Jack Sobon told me that he thinks purlin roofs were built when a board roof was to be used and rafters were used with shingles. Makes sense to me.

Looking at your example, Will, perhaps when the extension was built the owners had more money to build with and could choose the pricier shingle roof, hence the common rafters. Perhaps at the same time they even upgraded the original house to shingles, replacing the worn out (or perhaps just "low class") board roof.