The big networks filter such traffic, the small networks benefit from it.
https://www.reddit.com/r/legal/comments/1pq6kgp/is_it_legal_...
You may also get accidentally get your own website blacklisted or moved to a lower RPM tier, or provoke shadow-ban websites that you like to visit, or... generate more ad revenue for them.
Usually when it's brought up people say it doesn't work or try to spread fear that it is illegal. Google banning them but taking no action otherwise indicates to me and the thousands who use it that it is in fact effective and Google has no other recourse other than their control over the most popular browser.