In my experience, definitely functional.
I'm typing this on a KBT Pure Limited with aluminium case and added Hammer aluminium plate. They have both made a tangible difference to how the board feels to type on. On the opposite end of the spectrum is the Cherry G80-3000, PCB mounted board. It doesn't have much board support in the plastic case and feels utterly horrible to type on, IMHO, despite having PBT keycaps and the same switches as my Pure (MX Clears).
A heavy, solid case stabilises the board and gives a nice heft to it. A plate increases the rigidity and the solid feeling. Feels more precise and "tight". The whole board feels more like a unit and less like a collection of loosely joined switches.
Orings or trampolines help the bottom out feel and prevent "shock" on bottoming out. Almost mandatory for a good solid MX board.