I would guess that it would blow too much air over the heat exchanger, and would not provide sufficient warming. I don't think it would have the ability to cool the water in the cooling system too much that it would impact engine operating temp. I'm pretty sure that you won't hurt anything by running the fan on high with heat.
That's a great answer, Bryan, and is probably the thinking behind the restriction. Whether or not it could drop the engine temp depends on the temperature of the ambient air and the size of the heater core. I've been in cars where you could only use the heater in recirc mode on very cold days, especially while cruising at highway speeds. And, yes, you could watch the engine temperature drop if you didn't switch to recirc.
Modern ECU's are very picky about operating temp. You can't replace a 192 thermostat with a 185 anymore, or you risk preventing the car from ever running in closed loop.