Actually there's a few additional reasons as well. I did a buttload of research into what oil I could run in my Night Rod and wound up with a good deal of info. One of the main reasons that diesel engine oil is advised is that it doesn't have the anti-friction additives present in passenger car oils. Anti-friction additives wreak havoc with wet clutches, often destroying them before the oil can be changed out.
Also with a common sump like the V-Rod line your engine oil is also shared by the transmission. The shearing forces forced on the oil as it attempts to lubricate the gears would break down it's viscosity if it were regular passenger car oil, diesel oils have more resistance to viscosity breakdown.
But really, it's too easy to find the synthetic V-Twin motorcycle oils made by Lucas, Royal Purple, Mobil 1, and the MOST inexpensive, Valvoline. I ran Valvoline in my Night Rod Special and the clutch tightened up nicely. And it was $7.59 or so per quart (FULL synthetic). The MoCo Syn-3 is approaching $13.00 a quart, that's ridiculous. I won't pay it.