Thanks for the feedback, and for the honest take.
On the advantages, here is where it comes from for me:
I have a huge rack in my studio and another huge rack at home. Every time I buy a module I go back and forth about which rack it should live in, and whichever way I decide, sooner or later there's the moment where I sit in front of one rack and wish I had that one module from the other. With cassettes, that's a few-seconds move instead of an evening with a screwdriver.
And the time-consuming decision part disappears completely: which modules do I pull to make room, how do I fill the gap that leaves behind, and what else has to be rearranged to make it all fit.
I've also often wished for a very small rack to learn a single module in isolation. Ideally not sitting in front of a monster system, but small and tidy on the desk in front of me. I picture a little two-slot rack: one cassette with a handful of essentials, the second cassette holding exactly the module I want to dig into. Swap the second cassette, learn the next one.
And a few friends occasionally bring parts of their racks over when they visit. If their cassettes are compatible with my frames (which is what they're aiming for), it becomes very easy to bring whole cassettes along, or to lend them back and forth for a few weeks without anyone unscrewing anything.
So yes, functionally a cassette is close to a small powered case. The difference is that the "case" part is shared and stays put, and the moving part is only what has to move: rails, busboard, modules, no PSU, no cheeks, no cables to plug.
Your points on the hardware are well taken. Bent perforated sheet instead of the L profiles is a good thought, stronger and gives shielding and airflow in one part. Aluminium cheeks and levers would definitely look and feel better; I've started with printed parts because every dimension is still changing, but that's exactly the kind of upgrade that makes sense once the geometry is settled.
Your flip case idea is fun and creative, I like the chalkboard picture. How well it can actually be built is another story. :)