Since we now have this issue merged into symfony symfony/swiftmailer-bundle#64 the cookbook entry for sendig emails from the console is no longer correct.
http://symfony.com/doc/current/cookbook/console/sending_emails.html#using-memory-spooling

It should mention that in newer versions of symfony it is no longer required to do this manual flushing of the SwiftMailer cache if you have in memory spooling.