I think that this has sense only for sites daily consumption - well, that is, those where people spend every or nearly every day for long hours. For example, mail service, social networks, ebook reader, something to do with the work, Wikipedia, Google, github, Headhunter, etc.
For all other cases, the person once and there is no need to understand the intricacies of your interface. If you really really need the problem solved at the browser level (for example, UC is a function) or OS (Makosi), and not site - even assuming that these plugins often work no matter. Simply because if I go to hundreds of sites, I don't want to configure all of them individually.
And just, again, for systems with constant long hours of use can be justified. I think these sites each person no more than 3-5 pieces.