What @RK said. The temperature sensor has an accuracy of only +- 5C. The resolution of 0.25C is also quite bad for typical thermometer applications. Refer to the Temp Electrical Specs.
The temperature sensor is actually used by our Softdevices to determine if the low frequency RC oscillator needs to be calibrated. However, to determine this you only need to know if the temperature has changed, not the actual exact ambient temperature. Source.
