Hello all,
This was actually my original intention, however this being a RFC I wanted to show off different possibilites here (admittedly maybe then to the detriment of the as-simple-as-possible approach). However, I’ve since added a modifier key check to the patch set to showcase both possibilites; holding down the Modifier key while Mid clicking now shows the extended menu as per the screenshot above, while Mid click without Modifier brings up only the user folder menu as per below, making it more like a personalized shortest-way-to-favourites (more on this below) feature. No sub-folders are (currently at least) parsed for this one either, but maybe this is actually the dead simplicity one would be after in a case like this?
See above - “By the way, for people lacking a three button mouse, the same action is also mapped to Modifier+Right click (cf. how the regular Right click context menu is also mapped to Modifier+Left click).” Also, I would argue that most relatively modern laptops probably have touchpads that support multitouch, and then often also e.g. one/two/three finger tap actions simulating left/right/mid clicks respectively (of course, this capability may or may not extend to more niche operating systems like Haiku, so YMMV perhaps).
Well, the Tracker/Deskbar UX hasn’t evolved significantly the last 20 or so years, so it’s not like there’s a (widely known?) long queue of candidate proposed actions to assign this to, but hence also this RFC to solicit input from the community. As for the Desktop closely matching the File Manager (Tracker in Haiku’s case) and the two being closely (and arguably in quite messy fashion) mashed up even code-wise, this was a flawed concept already on other operating systems (cf. e.g. Windows Explorer), as these two UI elements are completely distinct entities UX-wise, as is the Deskbar. Yes, consistency is important where it makes sense, but (IMHO) not at the cost of UI element specific optimization of capabilities.
The core idea here is really to simplify some frequent UI actions by providing alternatives catering to different workflows. Sure, the Deskbar could really use some polishing UX-wise (IMHO at least), and perhaps also some adjusted defaults, but due to it’s nature and typical (default top-right) placement it requires more mouse movement back and forth on larger screens, plus the associated lengthier Deskbar menu diving. Contrast that with a “one click right where I am on the desktop” quick launch approach as per e.g. the screenshot above, which becomes more similar in reachability to a keyboard shortcut, but a mouse-driven one in this case (because different people have different workflows and so on).
To summarize, all in all, I don’t think there’s any harm in having a friendly community discussion - maybe at some point even a new Glass Elevator discussion? - around Haiku’s current (including maybe some of its defaults?) and possible future candidate UX design choices. What I provide with the patch set in Gerrit is simply a possibility for others to try out one such possibility for themselves, in a “letting code speak / try it out for yourself / seeing is believing” kind of fashion.
Thanks for the input so far - please keep it coming! 
BR//Karl aka qwilk/@xoblite