Comment 1 for bug 209376

The following document outlines the reasons why reply-to munging is a problem:
    http://www.unicom.com/pw/reply-to-harmful.html

With the existing setup, you've got the option of replying to just the sender ('r' in gmail, ctrl+r in thunderbird), or replying to everyone ('a' in gmail, ctrl+shift+r in thunderbird). If the mailing list software adds a reply-to header, then you can't easily reply privately to the sender (often requiring manual copy/paste of the address).