I don't see why updating everything that can be themed to be themed would be a bad idea, as long as it was done well. I know when I first started making themes myself I had to borrow bits and pieces from other themes until I really got a grasp on how everything went together. It would be nice to have a theme that incorporates as much as it can while still remaining usable and clean, even if it wasn't set as the default.
Maybe it's time to start working on a theme for inclusion in Rockbox as something to show off the available theme features..? Rather than updating cabbie to theme everything possible, it might be better to make a new theme and leave cabbie a little more simplified, at least until the code matures a little more and cabbie starts to look old.
As for cabbie, it might not be a bad idea to make use of multifont in some instances, and a base skin for the menus wouldn't be too bad an idea either since the old statusbar is rather tiny on the larger-screened DAPs. Anything else could probably be left as is as long as the base screen is set up correctly.