OK - the deal on antifreeze is this:
First, you need to consider that there are (currently) two types of automotive coolants available. Type 1, which contain silicates; and Type 2, which don't. There have been many, many documented cases of Type 1 coolants causing water pump seal failures in motorcycles. So, if nothing else - make sure that you are using a Silicate-free type. Regular Prestone contains silicates - Prestone Extended Life does not.
Secondly, even if you pick a silicate free type, you need to consider that each manufacturer creates their own blend of corrosion resistant additives. Some use additives that may not be compatible with other brands - so if you make a switch, you have to be sure to completely flush the cooling system to ensure none of the old stuff remains. This involves putting all sorts of acids and neutralizers into the cooling system, running the engine, and then draining and properly disposing of them - which sounds like something of a hassle, just to save a couple of dollars.
Lastly, there is the question of concentration. Automotive coolants are typically sold full-strength, and are then diluted by the consumer. Motorcycle coolants are typically sold pre-mixed, and can be added straight to the system. I'm not saying that doing the dilution yourself is necessarily difficult - but its another step to go through. Also a full gallon of automotive type coolant would theoretically yield 8 quarts of premixed bike coolant - enough for at least three complete fills of the V-Rod. Assuming you change the coolant every two years - thats enough to last for six years. I'm not sure I want to keep hazardous chemicals sitting around that long in my garage.
Sure - at an MSRP of $9 a gallon, the pre-mixed H-D stuff is more expensive than many other automotive coolants. But considering a) you really don't need to buy that much of it and b) all of the things that could go wrong if you don't do the changeover correctly - it seems that most owners would be far better off sticking with the factory recommendations.