For these reasons. I am generalizing from specific cases in my past: having used Drupal but not Joomla, knowing Java and C++ and object-orientation in general but not having used C# outside a brief school project, knowing early Perl 5 but not the changes up through 5.10, having programmed a bit in Ruby but never used Rails when the job description just said Ruby. "I can learn it" is never an acceptable answer when they can get someone else who already knows it well.