Sometimes it's needed to disable 64bit packages from 32bit Windows OSs for example to prevent binary incompatibilities. With the current implementation of installer framework, it's not possible to do so.
For example one 32bit Windows SDK can contain both msvc2010 and msvc2010-64bit Qt packages. If the user
has 32bit Windows, then the 64bit Qt packages should not be possible to install.
(if the user has 64bit Windows then it should be possible to install and run both 32bit and 64bit Qt's)
This functionality would make it easier to maintain Windows repository, because we could just prevent the user from picking the wrong binaries instead of creating two separate repositories for each architecture.