I am not sure if this is a Qt or driver issue. I had a look at orienteering compass application and it seems that there is some Qt property that indicates the calibration state (0.0 - 1.0) with 1.0 best and 0.0 worst.
If you switch to calibration page on that application it simply reads the calibration property while you move your phone in 8-digit like loops until 100% is reached which means that the bloody thing is calibrated. However if you just reel around the somehow continuous calibration gets messed up and your compass needle goes down South. Compared to my old Garmin device calibration is needed there, too but you deactivate it once the device says its enough so it stays calibrated until you replace batteries. It would be interesting to know how Qt calibrates or calculates the calibration property.

But back to the topic. I agree with you that compass bearing should only be activated by user if needed and not be the default setting to prevent battery drain and distorted compass calibration.