I can understand why they would want to make the easier for theme developers to upload themes, and have more safety measures in place since the learning curve is lower for themes than plugins.
But theme developers should definitely have SVN access if they want it. I don't even really think you should have to prove yourself, having your theme vetted the first time should be good enough, just like plugins. Makes sense not to give access automatically if most of the developers won't know how to use it. But if they take the time to learn how to use SVN, and ask for access, that should be enough of a barrier for entry.
I haven't contributed any themes to the repository yet, but now that I have sucked it up and figured out SVN to contribute a plugin, I know I'll have a hard time making the switch to the theme system.
As much as it is realistic, I'd like to see themes and plugins treated the same.