On your account page you can enable RSS feeds. The home page then has a link to the feed, which I have in Google Reader. It includes all file changes, as well as machine additions and removals.
The email notifications for System additions most certainly didn't exist when I wrote Dropbox about it. I did however, know about RSS but didn't choose to use that as a notification system (and wasn't aware it notified about machine additions or other system stuff).
I had perhaps a dozen or so old entries on there. Now, I don't know if there are actually any serious security implications here, especially since most of those instances are genuinely defunct. However there's no sense in leaving them around if I'm not using them anymore. Maybe it's worth checking out and pruning? I don't know.
On your account page you can enable RSS feeds. The home page then has a link to the feed, which I have in Google Reader. It includes all file changes, as well as machine additions and removals.