Given that there's a good variety of bluetooth handsfrees available whose whole point is to sit on the dash and send audio to your FM radio, it seems likely many people want just that. If you're on the highway, only a few cars traveling with you could reasonably catch more than a brief burst of speech.

Denying the benefit of this as an option is much like denying speakerphone -- sure, it reduces security (especially with a parabolic mic ), but it's sometimes worth it.