The problem is most people who live where it can get quite cold in the winter will not open a window before taking a shower and leave it open for the 20 or so minutes required to vent most of the humidity. When we bought the place we live in now there was minor signs of surface mould on the ceiling where it meets the walls. The first thing I did was install a vent fan and vented it out under the soffit. We have been here 3 years and there is no signs of mould in the attic - I was up there last week installing a box for a roof fan... I prefer to vent warm air out the bathroom rather than freeze my assets off after a nice hot shower.
I still want to install vent fans in the other two bathrooms, but since they are not used as frequently I have not been in much of a hurry to complete the task.