"And this seems like a waste of the only paired characters Arc doesn't use."
I don't think even the  syntax really pulls its weight. Between (foo [bar baz _]) and (foo:fn_:bar baz _), the latter is already more convenient in some ways, and one advantage is that we can define variations of (fn_ ...) under different names without feeling like these variations are second-class.
(Arc calls it "make-br-fn" rather than "fn_", but I wanted the example to look good. :-p )
Personally, I've never gotten comfortable with the colon intrasymbol syntax. Even in your example, I'm having trouble parsing it right now. I really don't like how it makes "bar" look like part of the first part, not the second.