WOZ will be updateable via network/WiFi/Internet, so there is no reason to have to physically access the board - especially not the actual manufacturer. And even if you don't have a network where you are and must rely on USB... then why not run a $0.99 USB extension cable from the back of the motherboard right up to the coin box?

Blew your brain right there! ;-)

Reasons I can think of for computer box in cabinet:

1) (The Biggie) Not enough room in backbox with LCD also in there (it's fairly deep).
2) No need to run mass quantities of cables up through the connection between cabinet/backbox, allowing for easy backbox removal.
3) Allows for very easy disconnection of playfield from electronics and vice versa.
4) Permits shorter wires (money savings).
5) Keeps center of gravity lower in machine (no extra heavy backbox).