Running this mining script on the computer of visitors of your website is really simple so it's no surprise that the practice is spreading. Some sites are doing it intentionally, while others got hacked or are unintentionally running it via an ad network. The main problem is that this new monetization method results in high CPU utilization, when no throttling is used it results in 100% CPU load.
Malwarebytes dedicated a blog post to the issue of drive-by cryptocurrency mining and reports they're now blocking an average of 8 million drive-by mining attempts per day. The practice got a bad reputation but Coinhive aims to turn the tide by pushing it as a new way for webmasters to earn money, without plastering websites with ads.
To give you an idea of the scope of drive-by mining, Malwarebytes has been blocking the original Coinhive API and related proxies an average of 8 million times per day, which added up to approximately 248 million blocks in a single month.
With their new mandatory opt-in API, Coinhive hopes to restore some legitimacy to the technology and, more importantly, push it as a legal means for site owners to earn revenues without having to worry about ad blockers or blacklists. This could also benefit users who might not mind trading some CPU resources for an ad-free online experience.