There are still plenty of programs that write vast amounts of information about themselves into the registry and that also install components in the sub folders of the Windows folder, all the registry information and and specialised components are lost when you wipe the OS partition, so generally you will still need to reinstall anything other than the simplest standalone executables.
However it is good general advice to adopt a strategy like this, it is also worth recommending a large OS partition; on a 500 GB disk I would allocate 80-100GB for the OS on a Windows 7 installation but then again I have particularly heavy space requirements on my systems.