Agree with all this @iogui
Even more, some ideas need to"mature" a bit in one's mind... It happens often that you thin the best way to attack an issue is a certain way, but thinking better you find it's actually not the best way. Writing these ideas down on this forum helps a lot with that
The basic assumption - there is a "panel" object that is the base building block of all the furniture WB - seems very right to me. It might not cover 100% of the needs, but it will cover 99%. Which is efficient enough
The remaining 1% is therefore indeed covered by allowing objects from other workbenches to be used in place of panels.
The panel object can start with something very simple, like the arch panel or the partdesign pad... You have one profile (or none, maybe just length and width properties), and you extrude it by a certain thickness in a certain direction. That would be enough to start playing.
Then the whole big problematic issue will be how to arrange the panels automatically. Indeed the first thing that comes to mind is a kind of assembly module. But as I said before, I think it might be overkill... Panels in a cabinet don't really need "free-form binding". On the contrary, they obey a pretty strict set of situations: side panels, back panels, top panels, shelves... If you think that someone would need to design a cabinet using a complete assembly module, and defining by hand the constraints between each set of panels, that would be a lot of work and not very practical...
Another idea crosses my mind now: Imagine drawing a cabinet "diagram" by hand, with the sketcher? Something like this:
Then you'd be able to "attach" panels to a certain edge of the sketch... That could be an interesting way to proceed I think. Of course there we would have the problem of toponaming... That would need some serious testing.