Most of the feeds I read are HTML excerpts with a link to the full article, or notifications with a link to a page. It makes perfect sense to read them within a browser, as I do in Opera.
Note that I didn't say you shouldn't read your RSS feeds in the browser, just that it isn't necessary for it to be part of the core browser itself. Extensions, web apps, etc. are all ways of reading RSS in the browser without it being directly incorporated.