Since you're already typing commands to the server, why not use wget and unzip to download themes and plugins?
That way there's no need to download them to your computer before uploading them again to the server.
Also, with the right permissions, casual users can update everything by clicking around in the admin interface. There's no need for them to dabble with this stuff.
The whole idea is type minimal commands. With wget, things get out of hand when it comes to updating plugins.
I understand that everything can be managed via the dashboard, but thats not the case when it comes to some servers that do not allow Safe mode to be disabled. Also, FTP connections are insecure.
I still see potential in Subversion, however with some streamlining.
There is a move towards Subversion among the more advanced WordPress users. I am trying to understand whats in there for the casual user and how it can be improved.