They keep a copy of all your data on their servers. Not just a copy, but the change history of every file so you can revert to previous revisions of individual files.
Ah. How unfortunate. So because they chose to change Dropbox into some repository/history thing, instead of just file-sharing, now they have to charge everybody.
Dropbox didn't change their business model to a "repository/history" type thing. It has been this way from the very beginning.
It sounds like you just didn't understand the original intent of Dropbox and were using it for a purpose that it is not entirely designed for.
Dropbox syncs all of your files with their servers. Lets say you have 2 computers with Dropbox installed and under the same account. You can add FileA to one of the Dropbox folders. It will sync that file with the server and then all Dropbox clients sync with the server. This is useful in that you could format both computers and lose all physical copies of FileA. However, as soon as you re-install Dropbox it will sync with the server again and download FileA from Dropbox.
It has always been a lightweight and easy to use cloud-backup solution. The collaboration and shared folders have been just sort of a bonus.
If you want to just keep several machines in sync without the middleman then you are going to need to look into other solutions. Something like rsynch might be a good option.
Dropbox's feature list calls out File Sync and File Sharing as the major features. Forgive me if I took that at face value.
"Sync Windows, Mac and Linux computers" sure sounds like what it says.
In fact the word "server" doesn't occur until the 18th bullet point. If this is a the major feature point, they need to talk pointedly with their marketing guy.