although, as I've said, I don't recall having seen such an arrangement. I suppose the main problems are (a) in finding the fan switch to operate it night before one switches the light on and (b) remembering to put the switch back into the 'on' position (again, if one can find it!) after switching the lights off at night.
I also haven't ever seen such an arrangement, in my friends houses I've been to, they've all had a single light switch and a fan isolator switch. Most of the time (at least for the people I know) the isolators for the fans are off because the fans don't seem to do very much apart from make an annoying noise..... Just double checked and asked my fellow work colleagues about their arrangements and none have a double switch arrangement, all have the usual single switch arrangement with isolator for fan.
Edit: If it really stinks in the bathroom I just open the window for 5 minutes; does a great job of clearing a bad smell unlike the useless extractor fan
Thanks,