I drive an expensive car and I use the Bluetooth. I ride a bike to work three times a week when I can, 10 miles into the centre if Liverpool on busy roads, including a few hundred yards at each end of the M62 which is dual carriageway but treated as motorway by people in cheap and expensive cars alike.
I share the views of a previous poster. Twenty years ago we didn't have mobiles. In the last 20 years, despite the massive use of mobiles in cars that the law eventually reduced, we've not seen huge increases in crashes have we.
There are all sorts of dangerous things to do in cars when driving. Radio has been mentioned, changing cd, opening cigs and lighting it, opening canned drinks, reading maps, playing with satnav etc.
There is a law to cover all of that anyway, but it has to be proved that your driving with undue care or recklessly. There isn't a law of smoking while driving, changing cd while driving etc.
The law for mobiles is a cynical income generation scheme. You can be prosecuted for holding it even if its switched off. Guilty, no defence.
For what it's worth I used to regularly use them, and was prosecuted in the pre points days for the offence. I did have an interesting debate with the police officer, who was up for it to be fair, about his multiple in car communication equipment and road use. I drove an Astra at that time.
Fwiw, the most dangerous drivers are people in old cars who want luxury cars and are bitter, people in luxury cars who get their income through criminal means and women in a morning applying cosmetics. Particularly the latter. I apply that to both my cycling and driving.