If you do not provide a branch for your environment, you'll never have any semblance of a working progression through development and testing.

This line really jumped out at me because I see it as completely the opposite. Source Control on its own has no way of enforcing that progression, so you can't trust that it always maps 100% to a specific environment. Unless you have a very tightly controlled environment (e.g. you develop a SuiteApp in a single account where only you and your team have access), there are just too many people that can at any time change files/objects without going through or even knowing about your source control. If it works for you and your team, that's awesome and you've no reason to change it. However, working with many clients, especially those who have very knowledgeable or active admins, I have never been able to trust that

master

always 100% matches Production,

develop

always 100% matches Sandbox, etc. I've taken to completely divorcing source control from environment status in my mind.