As I said there have been times, seen in these forums, when the first instance of a hidden/undetected piece of malware was the presence of outgoing spam mails.
If the user has no email client, then no resources would be consumed, so there really no mega gain of not having the shield active. Not to mention the options given in a custom install, minimal install. Plus there are a number of components that I would suggest that could well be removed as I wouldn't consider them security related.