Nearly all desktop applications use a fixed header, it has fine usability there. For a web app, as opposed to a primarily informational site, I think there are definite cases where a fixed header is preferred. Gmail for example, it makes sense to have control butons for your message always visible.
tl;dr on your average blog I agree, but in a web app I don't.
Since lukifer wrote "in most cases" you're only disagreeing if you think there are more web apps out there than blogs or informational sites. He didn't say a fixed element is never desirable. I think you're agreeing.
tl;dr on your average blog I agree, but in a web app I don't.