The discussion was on the pitfalls of the user not knowing something was sent, and one of them is data loss. The fact that a web app /can/ ameliorate a lost connection is orthogonal. If the user was told the email was "sent" he's going to feel free to close out all instances of e.g. Gmail, so that clever Javascript isn't going to be running. If -- if -- he re-uses this same browser in the future, which is not guaranteed since he thinks the email was sent and may have no concept of browser storage anyhow, the email may send then, which could be hours, days, weeks or months later, unexpected behavior which will cause unexpected results.
