I get exactly what you're talking about. If the thermostat is stuck closed, you're not getting the coolant in the radiator to the engine, so it wouldn't matter how cold it gets.... you're still going to get hot if the fan is or isnt running.
My dad was the one who told me it could be bad if the thermostat stuck closed and I really didn't think too hard on it (he's a mechanic.... but usually diesel, so he's been known to be off when it comes to newer cars).
And I wasn't sure if people actually used a switch for their fans (I'm sure some might) but I'd rather not. In fact I had a setup like what you're talking about at one point, I'll have to see if I can't find it. I don't recall actually installing it.