Okay...maybe I am missing something. Maybe site mining (coinhive and other similar) has its merits...but I can't see them doing anything good for anyone but site owners. I can see how it does lots of harm though.
Arguments that it will end ads are naive. It will just be added to the list. Ads will still be there, there is no incentive to stop selling ad space. If there was a rule that said, "this site mining protocol can't be used on sites with ads"...then yeah it would cut ads. But, such is not the case.
It is worse than ads since ads are only a visual/mental drain and site mining is actually costing users money and connection speed. Ads require almost zero data/bandwidth and are essentially cost free. Site mining forces site users to actually pay for extra data, bandwidth, and power. If site mining becomes as standard and abused as ads are...this could effectively raise our internet/phone bills quite a bit.
Everyone hates ads...except the people selling them. Even the people who use ads hate them...but their profits encourage them to use ads anyway. So, everyone is going hate site mining too. Being associated with it is not doing XMR or cryptocurrency any good.
Another harm is that it potentially enables a large portion of mining to be controlled by a single entity. This is already an issue with big mining setups...and lots of altcoins have attempted various strategies to combat this. Most, if not all, of these strategies could be circumvented by a dominant site mining protocol's creator.
So, why is it that everyone is so supportive and excited about these site miners?