One issue with 3rd party blocking is that it is difficult to tell if that other server belongs to the same owner, or a different one. Just a few days ago, i stumbled over a situation where scripts necessary for the site to function, where pulled from a different server - which however had the same owner.
So, whilte for example blocking of 3rd party script domains may be an efficient start, its not perfect - you will sometimes block scripts from the same party on a different domain. In that way, what this "3rd party filtering" really means, is filtering of content from other servers than the one on which the request originated - and sometimes not even that.