The big advantage over built-in car navigation systems vs hand-held GPS units is that they measure speed and distance far more accurately via the wheel sensors. If you live in a city with a lot of tunnels or GPS dead spots, this can make a real difference.

I would imagine the same information applies. If you can blend the GPS and the wheel sensors, you would get the most accurate reading, because wheel sensors can become inaccurate through fitment of different size wheels and/or tyres.