Interesting that some commenters here seem to have an attitude of "let's build an app for the blind", "there is no hope for the blind" etc. I'm born blind and build software and do some accessibility consulting for a living. Fortunately we've reached times where the blind community can build software themselves.
An excellent example is NVDA[0], an open source screenreader for the Windows platform, mainly build by two blind guys. With Firefox, I think it is the best combination for browsing the web these days, even outperforming commercial screenreaders.
Oh, and if you want to know why semantics are important, grab a screenreader and read a HN discussion. The lack of nested lists and/or headings to indicate the relationship between comments makes it just a stream of comments without any structure. If proper semantics was being used, I could for example skip an entire subtree of the discussion if the starting comment didn't interest me.