I agree, i cannot see most webmasters integrating this tag. There might be a 20% penetration, and this being from tech savvy blog owners. Those are the owners who for the most part know how to avoid the spam in the first place. People with personal homepages will remain unaware on how to implement these changes, or even that they should. Most of the blog is on old abandoned blogs so it nullifies everything anyway.
For the long term future it might help. But for the short term, it’s not doing much.