I liked them, when I was scraping Directgov pages in 2010. I still like them, in much the same way I still like XSLT. In retrospect, a useful tech that didn't have great tooling adoption (especially by your socials) and so is now all but legacy except among those of us who still wish it was spoken.
A bit like speaking Cornish, but for your <head>.
Were you to add them and I was to scrape your page I'd happily use them, but I think I might be Special.