On the Etymological Fallacy

I never understood why people are so fond of the argument from etymology (aka the etymological fallacy) - that where a word comes from necessarily tells us something about the word itself, or even its application in a larger context (see: eco-feminists and their rubbish etymology for "gynecology").

A deliberately facetious example is the familiar quip: "The word ‘politics’ is derived from the words ‘poly’ meaning many, and the word ‘tics’ meaning blood-sucking parasites."

A good counter-example to the argument from etymology is given by Language Log: claiming that Arabs cannot be anti-Semitic because Arabs are themselves Semites. Yet nowadays "anti-Semitic" does not mean "being against Semites", but "being against Jews".

Given that language is a living thing, and that meanings become obsolete and new
there are no grounds for using etymology to gouge out hidden meanings or significance from words, beyond it having appeal as a cool party trick.