In my 30 years of electrical experience, I have yet to see a bath fan NOT on a switch of some kind. Most of them go on with a light in the bathroom. I myself put them on timers, because my kids never remember to shut off lights and such.
Timers are better than switches because you want the fan to run after the shower / bath to allow all the humidity to exhaust out. If its on a light switch you only get ventilation while you are in there.
Somewhere I read or was told it should be running for 30 minutes after you get out to bring the humidity back to normal.