It’s changed everything, but not all this instant. The vast majority will adopt the tag basically right away. There will be no reason for new blogs to not follow suit in the future, unless the blog exists merely to spam links.
It’s a concrete, no-brainer positive with zero downside, even if it doesn’t fix every problem. If even one blog comment spam was ignored, that would be a positive.
“Then when you add in the rest of the webmasters who don’t run blogs, but see this as a way to preserve their own PR… bang!”
Honestly I can’t imagine how you can see this as a negative. It is a tremendous thing that will lead to a great diminishment of the useless, link vomit on the internet… and I really like this… it will mean less of those idiotic automated exchange links email spams I get every damn day.