Adding the symlinks to /usr/local is an option in the OS X installer.  Although it is enabled by default, you can easily disable it in Installer.app by selecting "Customize" and then unchecking the install of the "UNIX command-line tools" package.

The main reason for installing the symlinks by default is, I believe, a historical one.  Using /usr/local/bin/ to refer to an alternate Python is an established custom.  On the other hand, as you point out, adding the OS X framework bin directory to the path is accomplishes the same thing and is more robust and, in fact, necessary if packages are installed that add console scripts (unless a symlink is manually created in /usr/local/bin for the console script).

With the likely ongoing requirement for multiple Python versions for many users, it would be good to have a better way to manage versions than the current rather simplistic approach, i.e. the installer supplied 'Update Shell Profile.command'.