I suspect that parsing a float (or complex) is a nonimplemented feature. According to my documentation http://arcfn.com/foundation-doc.html#coerce a string can be coerced to sym, cons, or int only.
Well, parsing a float is implemented, in the sense that read will parse it for you, so that's not the problem. I suspect it doesn't work because num is just a place-holder for "this kind of number is not yet fully implemented; don't expect much". It will probably die in a future release.
