That depends on your perspective. If you're producing and serving one ready-made video at a time, say for a video tutorial, maybe converting to multiple formats isn't much of a burden. If you're rendering videos, say for raytracing demos, and they take a lot of processing power to render and you have many of them, it's a significant burden to recode all of them multiple ways. Given that there is one way that is already well established and works reliably almost everywhere -- Flash -- I don't have much sympathy for HTML5 advocates who can't offer a solution that isn't at least as easy and customisable as what we've had for years already, nor for a certain brand of hardware that makes a point of not playing by the same rules as everyone else in the game.
I don't know what you mean by "customizable", but the easy solution is to just use Baseline H.264 with a modest bit rate. It plays on everything mobile, plays native in many browsers, and plays in Flash everywhere else. Ogg is unnecessary, because everything that doesn't support H.264 natively supports Flash.
> It plays on everything mobile, plays native in many browsers, and plays in Flash everywhere else.
H.264 seems to be the most widely supported format today, if you don't get screwed by the whole patent mess. A significant class of users do, though, if only because they use OSS browsers that are never likely to support a royalty-encumbered format. And of course, if you're distributing H.264 videos you probably need a lawyer one way or another.
As for customisation, Flash offers all kinds of possibilities that HTML5 video just doesn't, from custom controls to copy protection. Some people might not care, but they're probably the same ones who object to table-based layout and then expect you to write 20 lines of CSS and import three different IE-specific stylesheets just to get a trivial layout. Check out past comments from people who work at places like Google/YouTube.
