In reply to cp.

Interestingly, the Theme Review Guidelines that are used to review and approve Themes to be included in the official WordPress Theme Directory already require Themes to respect the presentation-vs-content/functionality differentiation. Directory-hosted Themes are not permitted to include "Plugin territory" functionality.

In fact, the Theme Review Team was somewhat ahead of the curve for this trend, and initially took quite a bit of heat for defining and enforcing a "bright line" differentiation between Theme and Plugin functionality; but it has been worth the effort, and then some.