I think the question is not how much you want bluetooth, but how much you want to play songs on your phone over your car stereo.
In the current world you can often plug your phone into your car by a 3.5mm jack and play music. In the brave new world, you can plug a bluetooth dongle into your car's cigarette lighter and 3.5mm jack, pair your phone and then listen to music. If needed, you have the "option" of hiding this awful mess under the dashboard and hard wiring it.
It sounds worse than cassette adapters to me, but I am not a phone manufacturer, so I can't do much more than bitch on the internet :)