Re:You forgot the most important reason: (Score 1)

Except that the file the Flash player plays (swf) is an open file format, there are other programs out there that can generate swf files. Two examples are Adobe Livemotion, and Corel Rave (not free, but alternatives to the Flash Authoring tool) and i suppose if you wanted you could write your own player to play them in and make it open source.