You can actually use all the libraries you mentioned (jQuery, underscore) from ClojureScript. What CLJS really brings to the table are a set of abstractions and data structures that make it easier to write strong reusable functional code. In fact, alot of underscore comes built into CLJS and I think much of what jQuery buys you can be handled as a good CLJS library (but more on that soon!)
Very cool!
We'll probably add dom selectors and/or manipulation and wheter it's the same library as dommy (github.com/prismatic/dommy) or a different one will depend on if the selectors borrow the same abstractions and syntax as templating.
That is really weird. If there's one area where Clojure feels a bit strange to me it is numerical computation. I haven't fully looked into it yet but there's the whole underlying Java idiosynchrasies issue : for example where you need to wrap inside unchecked-int to dodge overflow issue or the fact that you need to be careful to be sure to fall back to primitives if you don't want to kill your perfs, etc.
In addition to that a lot of intensive computation do work really best when using mutable data structure. Which is sharp contrast with Clojure's main philosophy that favors avoiding mutability. Note that it cannot be done, it can, but it kinda feels "weird".
Then your question gets weirder: you ask about the advantages of ClojureScript server-side templating when the article specifically mentions using Clojure on the server and ClojureScript on the front-end (if I'm not mistaken).