This is a good point -- keep aware of trends. But once you are into details of "subdomain vs subdirectory" for your blog, the returns are far below the benefit

This is a good point -- keep aware of trends. But once you are into details of "subdomain vs subdirectory" for your blog, the returns are far below the benefit for the vast majority of sites. Those kinds of differences only help in the case of a photo finish. If you are really unsure about some minor SEO trick, flip a coin and instead try to shave 100 milliseconds off of your latency. Google is not run by Dungeon Masters. It's not a trap. A search engine will only pay attention to some feature that distinguishes websites if it helps them improve the quality of results. And its in their interest to communicate the features they pay attention to.