Graham Christensen has released a Google Analytics plugin for Habari.
But. You have to put that bit of code in the footer template for each theme you use. That's easy to forget when you've been using the same theme for a while and decide to change it. With a plugin, though, you just activate the plugin and the code is automatically injected into the page template. You don't have to change the theme's code. You don't have to suffer the consequences of forgetting to do so.