This is still a relevant topic for AEM Forms. I'm guessing not that many people use it because its pretty evident. When the guideContainer is loaded, the common clientlib gets loaded, adding reset and overrides for things like inputs, buttons and containers, which is ok, if you only have AEM forms on your page, but usually there are things like headers and megamenus and container, which will get affected.
I'm still yet to find a way to NOT include this clientlibrary but haven't been able to, and a solution such as "Add more styles in another separate clientlib (?)" its not feasible and a waste of developers time.