I too have an issue with this and its called accessabilty.
There is an ajax based theme being developed for b2evolution and one of the biggest issues has been making the theme work for visitors that DO NOT have java enabled.
Believe it or not, not everyone surfs the net with it enabled.
For those folks, your site, and theme, is basically useless.
It is possible to make a theme that works for both js enabled AND non-js enabled browsers, I assure you, it's just more work.
Anyone else remember that idea of "degrading gracefully"??