Well, I can't speak generally to you being a blockhead or not.
The advice is because you have to get a GUID from someplace, and then keep track of them all. Using the computer account GUID is a shortcut, because AD is going to keep track of that for you. So you won't have to have another list of "Computer=GUID" anywhere. But it does assume you're in an AD environment, and DSC is otherwise not dependent on AD. This certainly isn't the only way to do things – but it's one way, and some people have been happy with it.
But this kind of all goes away in v5 anyway. Configurations don't use GUIDs anymore, they use a human-readable name, so you have a lot more flexibility and probably an easier time keeping track.
Clever though it seems to re-use already existing GUIDs for DSC purposes, I don't like that approach. From my perspective it's a sort of killing two birds with one shot and such approaches tend to bring undesirable side-effects, sooner or later. Once bitten, twice shy, you know 😉
Regarding the keeping-track-challenge I consider an additional CMDB field for the Computer Configuration Item class the best option to manage the configuration id/name to DSC node associations, at least in the abstract.
Yeah, and that's another approach some have taken. Others use a custom field in AD. Others use an Excel spreadsheet. I think moving away from GUIDs – especially with the advent of partial configurations – is the best solution.