Ugh. Please, read the other side of the "argument", I was going to say "research", but seriously, the information is 5 seconds of google-fu away: http://shaver.off.net/diary/2010/01/23/html5-video-and-codec...
They don't have to bundle a h.264 decoder with the browser — Google is the only one that does that with their binaries (depending on how many transcoding machines they had for youtube, they may have already been paying the full license fee), and they're nice enough to dynamically link with libavcodec, so if you build it yourself you can just symlink to your platform's copy, and get every codec under the sun instead of just wav, mp3, aac, vorbis, h.264, and theora.
Apple and Microsoft just link with their native platform libraries (QT & DirectShow), and use the licensed codecs they already licensed to bundle so your computer can function in the real world.
Opera's using GStreamer on at least some of their platforms, but I think it's with a private copy. GStreamer would be a great fit for Mozilla if they'd get their heads out of their asses: they could even bundle their whole own private copy with only their foss-wank decoders. They don't even have to make it fall back to the platform libraries, as long as it's possible to drop in more decoders. They could even implement the same 'a proprietary plugin is missing' thing they do for Flash, and just download the binaries from the upstream developers (most of whom are based in western europe).
Apple gets you to install Quicktime on Windows if you want video in Safari.
Any thoughts on why you'd want to support every codec under the sun (and pay licence fees for most of them) if the market externalities and network-effects lead to only one codec being used? Why argue for "choice" if you know that will lead to people being forced to support one particular codec?
You'd ideally support every codec under the sun by falling back to the platform's API. That "choice" would probably lead to people being forced to support one particular codec (h.264 for now), but it means that they don't have to pay for it. It also takes it out of the total control of the browser vendors — so it provides a path for Dirac/etc. to take off in the future.
There's also the ability for minority codecs to persist without gross transcoding (you can always fall back to a download link + VLC, which is what you had before). One of the worst things about the Theora debacle is that nearly all of the video out there is extra-lossily transcoded from more efficient files.