"the assumption that if Guests are allowed and added to Teams, they should have access to the information being shared (otherwise why add them?)" That assumption only works if all guests are allowed equal access to the information in all channels. Without a distinct visual cue, it is easy to enter information into the wrong channel. How many times have you seen someone post a message, then say "Oops, wrong channel" and retract it? I see this a lot. (For the canonical example, ask UK politician Ed Balls.) The ability to make each channel distinct would not completely eliminate this problem, but it would certainly be a help. It's also likely to help rapid identification of channels, rapid identification of important notifications (if the highlight color is used in the notification pop-up), and I bet there are a dozen other useful ways to use it that I haven't thought of at all. This is, as I said above, a symptom of a wider problem - that despite the wide diversity of the hundreds of thousands of users of Teams, there is One True Way to use it, and Microsoft knows better than the user herself what that is: One True Color Scheme, One True Font Size, One True Font, One True Layout, One True Minimum Window Size, One True Notification Sound, One True Sidebar... and so on.
Microsoft