Quoted from mbaumle:
Wasn't modular board design the whole reason William's chose to separate both the driver board and CPU board with that terrible interconnect board--even though they were designed to operate as a unit, and neither could function without the other? Its funny to see how commercial pinball want to subdivide their boards to replaceable units, where the home models wanted to keep everything down to as few boards as possible, as evident behind the backglass with the driver and cpu boards combined into a single unit, and under the playfield with all the switches, lamps, and related wiring soldered onto a giant circuit board.
To me, it makes sense to divide power supply/regulation, logic, and drivers.
If you have a coil that locks up and cooks the board, just replace the driver board. If you have a regulator fail, just replace the power board. If you have a logic problem, just replace the MPU.
The downside is more connectors and a larger single PCB. I'm not sure if it ends up being more expensive one way or another, or a wash. I know the larger you go with a PCB, the more expensive it is. However, back in the 70s and 80s, some games had some fairly giant-sized PCBs.