I'm afraid the various newsgroups' consensus is flawed. You can't simply state that ALL email scanning is a waste of time, there are too many differences to make such a sweeping statement. avast's email scanner is easy on resources.
Email is still the biggest method of infection. avast's email scanner uses a localhost proxy to scan the email before it lands in your inbox. So if it is infected it warns you and you can delete it so it never gets into your inbox. This has a two fold benefit, one it doesn't get established making it more difficult to remove and two, you are less likely to have your inbox deleted if you scan for virus infection after receipt of email.
Many AVs can't extract an infected email from a mailbox file (a collection of emails for that email folder), so it deletes the complete file.
So no I don't think they and especially avast's email scanner are a waste of time or resources. And much less painful dealing with an infected system or a deleted mailbox.