... That's at least three uses that go away if we consolidate Constructive and Progressive.
Here's a fourth, the wiki login form. You can [Log in], but you can also create an account (usually phrased as [Join <wikiname>]). They're both significant actions, so neutral for the second button doesn't quite work. So they take different colors, and over time a consistent color choice here and elsewhere should suggest the interaction distinction in the guideline, even though only a tiny fraction of users will ever be able to articulate it. The onus is on anyone who wants to eliminate it to prove it's harmful.