A common practice im afraid. Gatwick inbound has been diverted into Cardiff on more than one occasion due to technical issues with the Cardiff based a/c.
Outcome the passengers were told and refused to de-board the aircraft when on stand and then the authorities were called to have them removed. Can't imagine being diverted such long distance from dest and being on a coach for nearly the same length of time than the flight itself.
Has also happened many a time if one of BRS a/c are having problems and a/c is due to sit on the ground at CWL for a period of time.
I have to say although inconveniencing passengers on one flight looking at the overall picture im sure they are minimising a more lengthy delay which would have a knock on effect for more flights at another base.
Common practice with scheduled airlines but the aircraft usually travels to destination offload passengers and position out empty to a base that requires the aircraft re-shuffling the schedule to minimising delays is they key aim.