Now that we’ve established that plugins extend the functionality of WordPress, what about themes? The frequently add functionality right? Well, yes, but no. Plugins add functionality, themes handle global design elements. Yes, some of them include functionality (which, by the way, is a hotly-debated topic), but the core purpose of a theme and a plugin are two drastically different things. I believe that themes shouldn’t include any functionality outside of that which is required for their use to the website. In other words, a theme designed for podcasting may need an audio streaming or playing interface integrated tightly into it to accomplish its purpose. In this case, I’m OK with that functionality being part of the theme. The same theme, on the other hand, probably doesn’t require a custom page builder to function. This type of “convenience” functionality belongs in bundled recommended plugins, not in the core of the theme itself.