Some users would like the option to set their Email preference to be "Email + Notify" and receive an email and notification but marking the notification as unread. Only if they reply to a notification comment should the notification be marked as read.
This is primarily for users who may work on-site for part of the time and off-site at others. To enabled "good habit" they wish to work entirely from Phabricator when they can and use the read status of notifications to see if they need to take any action. When they don't have access to Phabricator (out of hours, travelling, etc), they want to be able to use email's to see if they need to weigh-in on a task.
Herald doesn't seem to provide the option to create a rule for new comments on a task and not sure that the 'send me an email' option would allow replies.