I have a central gas heating duct in the bathroom. it pumps air up past the towel rack which helps dry towels a little faster. there is an extractor fan directly above the shower (venting outside), but only in operation during showers. the clothes dryer also is also hooked up to an outside vent.

since putting in gas central heating, we've not had any moisture issues. haven't run the dehumidifier, or had 'crying windows' since. we even still dry clothes on racks inside.

based on my experience I wouldn't tell you not to put a vent in the bathroom... but what works for me, may not work for you.