This is currently possible using the Tag Me add-on and it is useful to be able to alert people by email that they have been tagged in a post. In Tag Me there is an option in the ACP to turn this off or on for the site, thus giving the choice to admins as to whether or not to use it. If enabled, users can then choose to enable/disable email alerts individually. This is something that members of my community, myself included, have found invaluable. It's also a great way of giving AWOL users a nudge to return to the forum without having to compose a Conversation. Would be great to see it in the core.