But how would that situation be any different in the presence of some kind of system Chromium install provided by MS?
Chromium updates often, so you don't want to run your client-side JS against any other Electron version than the one you wrote it for because they change APIs frequently, so you'll still have one set of DLLs for each needed Electron version.
Given that it's Chromium (and thus effort to get things running should be minimal), and most Electron app writers aren't pushing Chromium to the bleeding edge, why wouldn't they test on the one that ships with the desktop OS with the largest installed base out there?
Chromium updates often, so you don't want to run your client-side JS against any other Electron version than the one you wrote it for because they change APIs frequently, so you'll still have one set of DLLs for each needed Electron version.