But you can’t just remove them or keep them because some blogger or new book says too. You have to do your own research and understand for yourself what the best-practice is.

Etags exists only as an alternative to checking the Last-Modified header as a method of validating the freshness of a file to determine whether it should be re-requested and emptied from the cache, or whether it should stay in the cache.

Even though all proxies are supposed to be following a defined set of standards for handling these types of data, not all do. But one thing is true, if you make sure that you provide at least one valid way for the proxy/client to determine how long the file should be cached, everything else is redundant and was created as an alternative not as a way to clog up the net with unneccessary HTTP headers.