Moved my Minecraft directory to C:/Minecraft/.minecraft and edited all launcher profiles to use this directory. Deleted everything inside the old directory except the launcher files. After starting the launcher, it instantly starts downloading assets and libraries to the old, now unused directory, creates an empty versions folder and claims to download a new 13w23b.jar to the old directory. After hitting "Play", however, the launcher starts 13w23b from the new directory.
You can easily test it when using different texturepacks in old and new directory, the launcher always starts the game from the new directory (as setted in the profiles), but requires to download unused files to the old directory.