The weekly summary e-mails seem like they can be a part of the ground functionality of a forum, as they can help to engage new members by notifying them about what is being talked about.
I also agree it could be a good improvement for there to be a notice about those and choice given to new users about being subscribed to those in the registration form.
You may need to create a feature request topic so people can vote if they are in support of that feature.