IMO Kevin's initial paragraph of the PAPA rules that we use for the IFPA/PAPA unified rules represents why we do what we do. The experiences that we have had over the years has made these rules a constantly evolving set of rules, to try and make rulings that are as fair as possible for the player(s) impacted.
"The unique charm of pinball lies, in large part, in the physical nature of the game. Unfortunately, this means that unusual events and outright malfunctions cannot be prevented, nor can they be perfectly compensated for. PAPA attempts to strike a balance between compensating for malfunctions and accepting the physical nature of the game."
IMO it's a bad rule because I don't feel like getting punched in the face by players for situations like this:
Player - "Hey Josh, I plunged to the top of the KISS lanes up on CFTBL and it's stuck on the gate at the exit of the plunger lane"
Me - "Well Player, that's too bad, here are your options, you can shake it out yourself and tilt, or I can drain your ball for you"
Player - "Expletive, expletive you expletive expletive."
This ruling doesn't follow the nature of: "attempting to strike a balance between compensating for malfunctions and accepting the physical nature of the game."
Most rulings try to get the game back into as close of a state as it was in pre-malfunction (within reason - resetting game state features, etc). Penalizing a player with a loss of ball for a ball being stuck isn't even close to getting the game back into the state it was pre-malfunction.
The snake on Metallica is a timed feature, so that's a non-issue.
In looking back at my notes when we chose to allow Dirty Pool on AFM to be 'play on', was that there is a specific software feature in the game that recognizes when the game is in exactly this state.
Back in the rgp2 debate Cayle brought up Suitcase MB on 24 as a similar example. The game has the potential to put the player into a 1-ball multiball situation, with the goal of needing to make a shot to get back into 'multiple balls in play'. In 24 you're welcome to avoid the right ramp at your leisure and shoot anything else you want. In AFM you are welcome to do the same . . . for now . . . until we change it . . . then you can't . . . then we'll change it back . . . so you can . . .