Great idea... love it for both the theme directory and the plugin directory. Whenever I search for plugins, it's confusing when a certain plugin has had more downloads than its "competitors" (those that claim to do the same or similar functions) but it hasn't been updated in several months or a year. You had no way of knowing how many people were still actually using the product, versus how many had since abandoned it for something better. There have been plenty of times I've downloaded a theme or plugin, but almost immediately didn't like it, and thus, I got rid of it pretty quickly... so I can't help but wonder how many others did the same thing. Active installs is definitely more helpful.

WP should definitely credit child themes toward the parent theme's count. A child theme might simply be a matter of two or three lines of code to preserve certain tweaks from being lost when the parent theme is updated. The parent theme shouldn't lose credit. If the child theme's style.css has to declare the parent theme's folder, it should not be too difficult for WordPress's theme directory to determine the parent and give credit accordingly.