Both themes and plugins are definitely something that is a user-level function, simply because not everyone has the exact same tastes or needs. What’s interesting is how much most people will go to incredible extremes with cluttering up both their theme and their site with way too many things they don’t need. On the theme side it becomes a visual overload to the point where everything is a blur, and on the plugin side they load so many plugins that the site will crash constantly and grind to a halt whenever anything needs to be done in admin.