Its a problem that doesn't exist, but will in a few years from now. Those billions of sites depend upon advertising revenue. As ad-blockers have become more popular, the rates that advertisers are willing to pay has fallen. Eventually we could see a majority of users block ads and sites would have to shut down for lack of revenue. This initiative can preempt that.
Also remember the overused line "if you're not paying, you are the product". This could lead to a web that doesn't depend on violating the privacy of its users to make a dime.
Fair points, although the fact that Google is offering this product makes your point about violating the privacy of users a bit moot (in this particular instance...not the solution in general).
You could still browse the web with Ghostify (or µblock…), and only load the Contributor tracking code when you want to support an article.
That would both minimise Google's tracking and prevent you from accidentally contributing to crappy link-bait content. Basically a copy of flattr on Google's infrastructure.
Also remember the overused line "if you're not paying, you are the product". This could lead to a web that doesn't depend on violating the privacy of its users to make a dime.