Indeed a very interesting development, I would definitely enable it on certain websites in exchange for no ads (I currently block all ads even on websites I support because of the possibility of malware and a general aversion against advertising).
The world's popular torrent download website, The Pirate Bay, has recently been caught generating revenue by secretly utilizing CPU power of its millions of visitors to mine a Bitcoin alternative called Monero without their knowledge.
Source:
http://thehackernews.com/2017/09/pirate-bay-cryptocurrency-mining.html