If you still plan to install Windows 8 old school style by booting up to the installation CD, then the added Cleanup Disk functionality from within the installation wizard will be a real treat. Otherwise, it is a small addition that is nice to have for most people who will be installing with Windows already booted.
According to Neowin, an updated Windows 8 Build 7955 (pre-M3 build) uploaded now features the new Metro-like installer. The features from the installer seen in past leaks continue to exist at this point.
Source: Updated Windows 8 installer now has cleanup disk feature | Windows 8 Center