[eric-hu]

Even if you had this 'lossy' tracking method for users, you can still deduce that the 'new' user generated by a browser is a potential match of the old user agent info with a certain probability.

More than likely, only the browser version will change. For larger updates, would-be-broken plugins would disappear or see a newer version. It would be a ton of effort to track users this way, but I think it's within the realm of reason for those with enough incentive (NSA, maybe advertising companies)

[Djehngo]

It doesn't need to be easy to implement to become easy to use, if someone implements this in a way which can be packaged up and included on sites with only a single line of code somewhere it becomes trivial to apply this to any site that wants it.