Alex, definline's intention is to be considered like defmacro in case of (my-definline x) where it saves one function call, but to be callable as a normal function in higher-order scenarios like (map my-definline [x y z]). Therefore, it expands to a normal defn form with :inline metadata which is a macro that's used by the compiler.
Wether it should be removed in the future is a separate issue. It is there right now, and people use it. The consequence of this bug is that you cannot trust AOT-compiled code, because there might be some dependency that uses definline. Additionally, all clojure.core definlines (booleans, bytes, chars, shorts, floats, ints, doubles, longs) are broken when applied as HOF, because clojure itself is always distributed in AOT-compiled form.
Really, it would be grossly negligent to release Clojure 1.6 knowing of this bug.
I've written a more detailed mail to the clojure-dev list: