Why do I get the feeling this discussion has become a lot like an Immigration / Border security debate? LOL
Personally, I concede that Nic has found a potential problem in the failure of one switch, that, for all OUR purposes, seems to offer no obvious reason for its presence. In hopes of protecting the potential of a melt down of any 1 or 2 of 7 coils, the "mod" seems reasonable to me.
But, back to the actual purpose of the switch. Obviously, the switch is contained on the TILT relay. Therefore, since the existence of the TILT function is to prevent some and many kinds of "Cheating", the addition of the switch had to have been to prevent some form of cheating that was either reported from the field, or "realized" by one of the engineers while reviewing past or current designs. And yet, none of our bright minds, can point a finger at a single apparent, specific, reason. Partly due to how the rest of the circuits involved don't appear to require the same protection. Nor was the "change" implemented on single player games.
In reviewing schematics from Big Chief (4P) through A Go Go (4P) and Casanova (2P), there was an obvious change in how they "Disabled" power in the games in regards to Tilt or the presence of a TILT relay. If you look at a Single Player Game like Hot Line, this was the era when TILTing ended your game. There is no TILT relay, only Game Over and Reset to control power to playfield scoring features. As was mentioned, with Big Chief (as well as many single player games) there was only a single TILT disconnect switch on what I will call the return rail (Common, not the fused power). It would be my theory that it was found this single switch was taxed with too much current to handle everything and may have required frequent cleaning and adjustment. Therefore, as was mentioned previously, the power rails were separated sharing the load between two different branches (Tilt Disconnect was moved to the Power rail (not common) and fed playfield style coils and relays. At some point, someone felt it necessary to add a disconnect to the relays in the head and our switch in question was how they did it. Not thinking through the scenario that Nic has presented that if this switch fails, it allows a score relay hold circuit to energize, thereby energizing other coils should a score drum solenoid not be allowed to move and hit its EOS dropping out the score relay.
I honestly don't think that the design engineers would run "what if" scenarios for the potential failure of every single switch contained in a game (I could be wrong). Obviously, they would look closely over and review anything in regards to extra credits, replays, or unwarranted scoring to protect the interest of the operator. And re-visit a design anytime something was reported from the field.
As to reports from the field, it would have to be a pretty wide spread problem to ever make it back to the factory. If a game failed on location, the tech repaired it. And moved on. Only if the same tech, had to make the same repair over and over again, or over multiple titles, would they probably make the effort to call their distributor or talk to the factory directly.
As to cut and paste design, I have no doubt, this was how it was done. Once a design existed, and it was found to be reliable and cost effective, it would simply be repeated from game to game without change or modification unless something came to light to warrant such a change. Hell, I found this out when I had a Fan-Tas-Tic body with no head. I had a NOS backglass. So, I junked a tattered Jubilee and found there was only the need to add a single wire to an empty Jones Plug pin to make the Jubilee head work 100% as a Fantastic. Even most, if not all of the colors matched up on either side of the jones plugs.