Heres the truth. If you get in a wreck, or if a wire gets pinched on a hot day when that grommet is nice and soft, the power wire will short to the body. It happens. You may think it can't, but you also don't know what a dead short is so you need to admit that you don't know everything right there and just say "it can happen".
When power and ground touch, your batterys terminals are shorted together. This means it has 0 or nearly 0 resistance between the two terminals. It will dump infinite current out at this point until it dies. You cannot shut a batter off like you can an alternator. For this reason we fuse, because while that battery is dumping everything it can into the wire, the wire has to drop all of the voltage. This causes it to become red hot. It will ignite carpet or anything it touches, or heat other metal it is touching so that IT ignites something flammable it is touching. If you were to get in a wreck, you'd want fuses so the car didn't ignite while you were trying to get out. Just because you went with small batts doesn't mean they can't dump a ton of current under a short. You want math? Look up the current ratings for your batts under a short. You'll see how high it is and understand your power wire can't handle it.