We are using an idea portal for a specific feedback channel on a beta product release. The customers submitting the feedback wish to be watchers so they can get an email on all incoming feedback instead of visiting the site regularly to find out.
When a user votes on an idea in the ideas portal they are subscribed to it and will receive email notifications when the status changes.
While this feature you describe is great it does not help the situation. The portal users wish to be watchers on the idea portal and receive all new ideas.