I don't think anyone is saying ONLY one format. However, OGG/Theora as the preferred format makes the most sense. It's the only one that has the potential to work on all platforms. I'm thinking specifically of the new ARM netbooks, as well as thing such as the PS3 (while a dead subject now) that never had a working flash solution.
Plus who can honestly say the flash player on Linux is good? It works ok, but its shit.