Presumably quite negatively if you use them to build everything. But there are valid places, the obvious one being whenever you'd construct a complicated HTML fragment manually using JavaScript strings.

I'm currently using knockout.js with jquery templates to generate a 'feed' component that has unique content for each user. The content also changes very rapidly so there is little point in a search engine trying to index any single version of the page. I was quite hesitant to go with the JavaScript templating approach at first (ie would normally have gone with generating the content on the server), but it has made quite a few things significantly easier.

As long as you're mindful that search engines won't be able to index any javascript-generated content, you should be able to consider the use of all these new client-side templating tools.