Here in this bright and beautiful year of our lord 2013, when-if-ever is a sitemap necessary? I'm asking here about xml or equivalent sitemaps aimed at search engines, not at visible sitemaps for the benefit of humans.
The book says sitemaps are to help the search engine find pages they might otherwise miss. When does this happen?
For comparison purposes: I used to have a pretty comprehensive sitemap. About a year back I said the heck with it and cut it back to only directory-index pages-- either files whose actual name* is index.html, or named pages performing the same function. So even if the server explodes and all sitewide navigation footers vanish into the ether, the search engine can still find everything.
* That is, ahem, the real name of the physical file. Not the URL.