Well, the purpose of an e-tag is to check if you have cached a specific resource, so by disabling all caching you prevent that attack. e-tags can potentially link your activity between different web sites, but they (alone) can't deanonymize you. The Tor developers sometimes have to make trade offs between security and usability. Turning off JavaScript by default would be safer but it would also break half the web. Most people wouldn't know what was going on or how to fix it, they would just think that TorBrowser is shitty software and stop using it. The Tor devs decided that it's better for people to use Tor with JavaScript than no Tor at all. e-tags are the same way. The Tor devs believe that the linkability threat is minimal compared to the potential breakage of turing off memory cache.