I agree. And not just brushes but swatches, symbols, styles, all libraries would be much more useful if they were "live" to a degree and could be updated and edited application wide without the need to save and load external files manually.
(you're not missing anything, nothing new regarding this in CS6)
A refreshing idea.
But wouldn't one then have to think about differentiating / providing application wide libraries AND document specific libraries ?
I can open and edit an InDesign Library in any document. As for "app-wide" or "doc-specific", I don't mind if library elements are automatically added to a document library as they are selected or used, but the inability to update external libraries, on the fly, makes maintaining them cumbersome.
This may be true for you, accepted.
But I'm not sure if that is valid in general. I'd vote for a careful implementation.