I'm not saying you are wrong but my experience is that sending a radio message does have some effect on nearby enemy vessels.
I have done a little experimentation and there is some evidence that sending a radio message can attract a convoy. I have seen escorts change course towards me after a transmission of mine in both stock and GWX. I'm not saying it is well imlemented however I can say I've definately seen behaviour that shows that there may be some sort of RDF being employed.
Note this post from Kpt Lehmann on the subject:
http://www.subsim.com/radioroom/showthread.php?t=103968