I have the choice between a notebook with or without bluetooth. In which case(s) does one use "bluetooth" ?

To use a bluetooth mouse.

Connect your phone; use bluetooth mouse/keyboard; quick and easy file transfers to another 'puter; use a bluetooth device to lock/unlock your 'puter (at least Linux can do this); stream audio to another device including a bluetooth headset; etc.

I feel Bluetooth is a must have feature.

I use it to connect to my cell phone to connect to the internet thru Edge/Gprs if my ADSL is down.

I also use it to avoid cables when transferring photos & videos from my cell phone to the computer.

Also transfer music to my mp3 player.

for me, the mouse - no nuisance of a cable; also send music tracks straight to/from my phone without the hassle of cables, memory card reader/adaptors etc. Useful for getting music off other people's phones too, and some cameras have bluetooth so can upload photos direct. Imagine the cost is minimal, why not?

I expect the addition of Bluetooth capability is really cheap/small additional cost, and the Bluetoth capability does come in handy depending on your situtation and adds a nice additional wireless capability. Get the Bluetooth capability.

YESyesyesYESyesyesYESyesyesYESyesyesYESyesyes

Go for the one with Bluetooth.

You may not know how to use it or what it's for yet but once you have a play with it you will realise how useful it is.