The counter-techincal argument is that those users already don't get h.264 on XP. So what's the difference between not having it because the browser doesn't let you use the system libraries and not having it because there are no system libraries? As presented, the difference appears to be that you aren't really getting the same browser on different OSes if there are dependencies on your OS and OS version.
I think the technical retort there doesn't hold a lot of water. After all, your OS probably came with a browser that isn't Mozilla-based which will gladly use native libraries for this kind of thing. Moreover, whether or not they decide to do this, the amount of work developers have to do to support the ridiculous WebM format alongside H.264 isn't going to change: you'll still have to have your content encoded twice and you'll still have to sniff out which version to show.
I think if you frame the argument as "why aren't we doing this?" instead of "why should we do this?" it becomes a lot more clear which course of action is the right one: the one that means a better experience for your users, which means better OS and hardware integration and better battery life when using your browser. Users plural may care about consistency, but a single user is much more interested in features and performance.