Now let me get this straight. Adobe has a spec for the SWF format, and they have lots of documentation on Actionscript, so producing some spec would be easy. But the problem is that the existing specs only describe correct behavior, but not the more important part on how to treat errors. Consider an example where the spec might say something like "height: Integer - the height of the current movie". So what happens when your code does height = new Object()? It's written down nowhere. In the current closed world, the solution is easy: the accepted behavior is what the Adobe Flash player does. So if you want to write an open Flash Player like Swfdec, you don't need a spec, you need patience and lots of test cases. Because there'll surely be a Flash somewhere that does height = new Object () or height = height / 0 or height = "Hello World" or...
A good example of how hard it is to handle the unexpected right is Acid2 for CSS. And in the case of Acid2, there even exists a spec about how to handle all the errors. (I was going to link to something I read by I think Håkon Lie about why having a defined way of handling errors is important as opposed to just aborting, but I can't find it in Google.) And error handling mechanisms are an important part of an implementation. The Mozilla team needed 1.5 years to correct their error handling. So if you figure out something new and exciting about Flash, it can easily mean you have to redesign a large part of your player. So it's important to know beforehand and should be part of the spec.
And while we're talking about necessary rewrites: A part that no spec talks about is implementation complexity. If a function is O(1) in the official player while it is O(N) in yours, you have a problem when someone calls it excessively in a loop. And then there's probably code relying on timeouts, data input or the phase of the moon.
Another thing I've been wondering about lately is the complexity of implementing a standard. I have no clue how Flash relates to SVG in complexity, but SVG has an open spec and that one is 4 years old. Do we have any SVG compliant implementations by now? HTML has a free spec, too. It took the Mozilla team 6 years from open sourcing to a release for their browser. So if a complete Free Flash specification started to exist tomorrow, would it take 5 years to implement?
An open Flash spec would definitely make Flash inch closer to World Domination, but it'd still take a very long time to make it really Free. Open sourcing the player would probably make that happen way faster.