Tons of advantages to doing it that way, including the ability to easily maintain multiple versions of both GHC and Platform on the same system and swap between them with a single command. Also doesn't clutter up /usr/local/ with binaries not managed by apt or dpkg.
More generally, update-alternatives is a godsend for Stable users. It's like RVM or RBENV in the Ruby world - lets you install and manage multiple versions of the same platform and easily swap between them with a single command, including the system version from repos if you want. It frees you from out of date software in the repos while still providing a system to manage the complexities of it all.
