Honest question: Why not Gambit directly?
That question to me amounts to "How is Clojure better than X," which is a pretty big discussion. I'd say the primary reason is social. I've done much of my personal programming in Clojure due to various contingent factors (working with Java constraint solving libraries, various SVG rendering libs etc...) so I'm comfortable with the language and know the community. I see ClojureScript picking up steam and I see native mobile as a natural piece of the server+browser ecosystem starting to form around Clojure+ClojureScript. I hope clojure-scheme fits that hole.
I definitely wouldn't try to make the claim that Clojure is fundamentally "better" on a technical or aesthetic level as a language.