I work for a company that makes Bluetooth chipsets that specifically target automotive Telematics solutions, and I can say that A2DP in general is not trivial. A headset designer is most specifically optimizing for low power, and so would not want to have to provide the extra processing bandwidth required for the SBC codec (the minimal codec spec specified by the A2DP profile). That's one big reason why stereo headsets are generally much larger (to accomodate larger batteries) and have talk times defined in hours compared to days for a mono headset.