I turn it off for a number of reasons ...

- battery life, as NY biker said
- reducing additional emissions - yeah, I get that it's miniscule compared to the cell signal, but why take in more than I need when I'm not using it? Same with the battery life really, even though the amount of additional energy is miniscule once I plug it in, why burn more carbon than I'm using?
- risk of viruses - also miniscule, but again, why take a risk when I'm not using it?

But mostly battery life. I would not be able to get through a day without plugging my phone in if I left all that stuff on all the time. GPS is obviously the biggest battery hog - drains the battery in four hours - but any wireless service will drain it, and the more it's searching for signal that it isn't finding (like when it's paired with your car but not in your car), the more frequent and stronger search pulses it sends out, and that depletes the battery WAY quick. Just like those times when you're in an area where your phone signal keeps swapping, and your battery goes dead in half a day when you're not even using it.

Anyway, the only time I even turn the BT on is when DH is out of town. (This should go into the age and fear thread I guess ... I never worried about emergencies when I was single.) That gives me the time I need to answer the call before it goes to voicemail and tell him I'm driving and I'll need to call him back. If it isn't him I can see who it was and decline the call. This is a practical decision based on when he's out of town on his ongoing and endless family emergency, if I couldn't see who the caller was, I'd be so distracted with worry that I might not even be able to pull over safely.