Sure, their interceptible of course. But everyone has a cell phone anymore, they need the number he's using. If they have just the slightest bit of knowledge that he's using a bluetooth device, a simple scan for bluetooth signals could give away his location (much less people use bluetooth with their cellphones than the number of people that have just have cellphones). Basically, unlike a cellphone, it's a risk that has no real payoff for him (I'm not denying cellphones are a risk, they definitely are).