Interesting question and as stated previously, you'll get many differing opinions. Here are my reasons for not using the same 20/50 Harley oil.
Any oils, as long as they meet the required spec, will lubricate you engine, primary and transmission effectively. The thing is, that many of us want the oil to do more than just lubricate parts. Take the transmission for example. Syn 3 works fine in there, but does nothing to ameliorate the noise made by the poor quality of the transmission. I use Redline Shockproof Heavy in my tranny because it lubricates very well, helps with smoother shifting and eliminates a whole lot of nasty noises.
I use Mobil 1 V Twin in my engine and primary and do so because I have more faith in the engineers at an oil company like Mobil than I do at some no name outfit that was able to bid at a lower cost than anyone else for the packaging deal to produce Syn 3.
Also I'll be damned if I'll pay through the nose for Syn 3 just because it says Harley Davidson on it. Harley builds bikes, but doesn't refine oil.
Last, but not least for me, I refuse to buy a product (Citgo) that I know is owned by Venezuela where the leader of that country does nothing but rubbish America every chance he gets.
These reasons may not make any difference to anyone else, but that just the way I see things.