100% disagree with useless as described above. Having rx battery voltage for one is quite useful allowing you to know when its getting near the time to swap out rx packs. The temp sensor is also quite useful as well as it allows you to get a good ballpark on optimal motor temps once your familiar with your tuning setup.

The rpm also can be useful for gearing. To be effective though you need to use the mount that measures RPM off the flywheel and not the "sticker" pickups off the driveshafts. WIth this you can gauge in combo with a timer the best pinion/spur combo for a given track to maximise the useful rpms of the motor your running. In the right hands and with a bit of time and patience it can make for a good tuning tool for the track. Its helped me dial in my LST XXL and RC8T quite well.

I agree though, don't just buy a radio for or based on telemetry. That being said though, Spektrum's DX3S/DX2S and new DX3R Pro are 3 excellent radios worth considering with the telemetry option.

Its all in the application and how its used. In the right hands telemetry can be a very good tuning aid(especially for novices), in the wrong hands(or an expert who already knows his setup like the back of his hand)it can be 100% useless.

I myself feel that you can never have too many tuning aides/checks and balances for racing.