Bluetooth headsets are considerably more expensive than regular corded headsets (or no headset at all!). They are also somewhat more complicated to set up, and you then have the added complication of having to keep their battery charged.
Firstly, whether Bluetooth or not, you should try and always use some type of headset with your cell phone. This will vastly reduce the amount of radiation that you're pumping out of the phone and into your brain - some scientists believe this radiation could be harmful, although studies to date have been inconclusive.
In some states and countries, you are not allowed to hold a cell phone while driving, and the use of some type of headset is mandatory. Even if it is still legal to hold a cell phone and drive, it is safer to avoid this whenever possible.
A simple wired headset is a satisfactory solution for many people, but it literally ties you to your cell phone. With a Bluetooth headset you can have your cell phone in your briefcase on your car's back seat, and still place and receive calls.