ETags are really aimed at a different problem, where an end-user/client wants to refresh content that they already have. They can help let a intermediate cache

ETags are really aimed at a different problem, where an end-user/client wants to refresh content that they already have. They can help let a intermediate cache know that content has been updated, but the cache has to be configured to ask for it. Plus, for dynamic content even the cost of calculating the Etag is not zero.