The usefulness of this feature probably depends on how many folders you have and how you use them. I tend to have the same folder structure open/expanded most of the time, but with the option to open others as I need them. This means for most of the time there is no need to expand/collapse folders or scroll, and I can work with the frequently used folders very easily.
I can change the folder structure that is visible periodically rather than having to all of the time, and I rely on RMM6 and my email client remembering which folders I have expanded between sessions.
I’m also a fan of folders that expand automatically when you drag a message to them as they do in some interfaces. For example, I can drag a message to Archive and then as I hold the message over archive this expands to show the year folders inside where I can then drop the message in to the appropriate folder e.g. Archive >> 2017 or similar.