I don't think we're getting anywhere. A standard OBD2 output lists 4,000 universal diagnostic codes. They're very precise in order to eliminate guesswork. The codes are stored in memory and retrieved by the mechanic by plugging in the OBD tool. Are you suggesting that the diagnostics are unimportant, that guesswork (i.e looking it up in a service manual) is actually the correct method? That looks like what you're saying. Correct me if I'm wrong.
Understanding how the systems work comes first.
If you can't follow the flowcharts in the manual, the codes aren't going to be that helpful anyways. The codes and other ECU data are simply additional tools to make your life easier and help you decide what you're going to test.
And there's plenty of times where there will be no codes, or misleading codes. Or the code might tell you that there is a problem, but not suggest where to look for it.
And, as others have pointed out a few times, you don't need an external tool to read the codes on these bikes. You simply look at the blinks and cross-reference the manual.
Motorcycle mechanics won't touch an EFI machine without the matching diagnostic tool. Absent a Kymco dealer, I'll need to have the Kymco diagnostic tool in order to get my Kymco serviced at a Triumph dealer, for example. If the Kymco tool is non-standard, I'll need to bring a "How-to" manual with the tool.
On the contrary, I've heard many motorcycle mechanics don't bother to get the diagnostic tool, when they really should.