Who's suggesting that you change the system default? If you want to make your python scripts run on a certain python version, set their shebang accordingly. Changing the default interpreter for system scripts because you want your scripts to run on a different version is crazy.

If your lamp is too bright, do you swap the transformer supplying the whole street's mains electricity supply for one with a different voltage and hope your neighbour's computer doesn't break, or do you turn your lamp down?

/usr/bin/python should remain symlinked to python2, and never python3: http://www.python.org/dev/peps/pep-0394/

But in any case, all scripts should reference the actual required version in the #! line.

"For the time being, it is recommended that python should refer to python2 (however, some distributions have already chosen otherwise; see the Rationale and Migration Notes below)."

  #!/usr/bin/python

  #!/usr/bin/python2.6

As long as that's done, and the packaging is done right, there shouldn't be any major issues for the vast majority of programs.