Works for me
For those that are suggesting "leave it on" that's fine, good for you.
If however you have any sort of security background or system hardening tendencies you will, by default, want anything you don't use turned off (particularly services that broadcast!).
Apples implementation of Bluetooth doesn't allow me to make my device "undiscoversble" and until it does I will only activate Bluetooth when I need it
Minor point really but thought it was worth mentioning as there's more motivation to turn off Bluetooth than just to conserve battery
Doesn't Bluetooth need pairing before you can use it? Someone can't just start accessing your phone through Bluetooth without you first saying it's ok?