I would like to suggest that if a user is subscribed to more than one newsletter - which is probably a common occurance - then all the newsletters will be aggregated into one e-mail. If I'm marginally interested in 10 different topics, that means 10 different e-mails, which is a clutter. Just send it all together and make sure the formatting shows a clear distinction between the separate sites.
I've suggested this on the newsletter feedback thread but it was suggested I create a new feature-request, so this is it.
Now, this suggestion is actually kind of orthogonal to this one; maybe the best option is to have this aggregation is a toggle-able option.