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YSlow is exaggerating with that. In fact, you have to carefully read their explanation to understand in which context ETags can cause problems.

Here's the executive summary. YSlow supposes that you have a mega-site running on a server cluster. In this setup, there are dozens of web servers serving the same site behind a load balancer which distributes requests evenly to those servers. The problem is that each server has the same files. Each file stored on the filesystem has an "inode number". Think of it as an address. On one server it will be X, on the other it will be Y and on a third server it will be Z. Same file, on three servers, three different inode numbers. The ETag is constructed by using the filename, its modification date and inode number. The filename and modification date are the same on all three servers, but the inode is not. This means that each request served by a different server will give a different ETag, forcing the browser to always redownload the file, which is undesirable. Therefore, YSlow tells you that you need to disable ETags.

Back to reality. You have a Joomla! site. Most likely you host it on a shared account, a VPS or -if it's gigantic- on a dedicated host. A single server. The ETag for any given file will always be the same on each and every request. Under this one server setup common in Joomla! sites you actually need ETags because they will hint the browser not to download the file again if it's already in the browser cache. In other words, in 99.999999% of Joomla! sites you need Etags. YSlow's advice is on the wrong context when talking about Joomla! sites.

As a result, I am never going to add an anti-feature, i.e. an option which will degrade the performance of your site. I hope that makes it clear enough :)