Media codecs are a very common source of security holes, which is one of the main reasons for not indiscriminately using just any codec that might be installed on the system. I think most users value the decision to not unnecessarily expose them to these security holes.
Also, that wouldn't change anything about the current situation: we still can't rely on users having the right codecs installed. We're not saying that Linux and Windows XP won't be supported going forward. They most definitely will. As part of this first small step towards deployment, we opted for a conservative approach where we only enable Shumway in situations we have the maximum amount of control over. This will be broadened over time.
