I think the only worthwhile argument is the native look and feel which is extremely hard to emulate using HTML5 (although not unfeasible, see http://pattern.dk/sun or sencha's fb client)

But on the other side it is also very costly to build multiple versions of an app on different native technologies and there HTML5 has an obvious advantage.

Speed is not much of an issue for most of the apps, hardware accelerated CSS can give extremely interesting results (again see http://pattern.dk/sun or FB client)

But overall I agree that doing well with HTML5 requires a good level of expertise.