I was chatting with Jeff about this on Twitter, but I thought I'd add my thoughts here.

As you become more familiar with the way WordPress works, you discover that there is very little real difference between plugins and themes. A plugin can easily take control over the rendering process, thus the plugin becomes a "theme". Similarly, a theme can easily provide plugin-like functionality (and many do). So, the definition of what is a "plugin" or "theme" feature is very blurry.

As a coder for iThemes, I try my best to not add features to a theme that is already very well supported by plugins. The reasoning for this is multi-fold: It reduces the complexity of the theme, thus reducing bugs and support needs. There's no reason to rip off a plugin author's work just to add a new bullet point to a theme's feature list. It allows me to focus on creating new features rather than rebuilding features that have already been done and done well.