Because the orange deposits build up on the valves, combustion chambers in the heads, tops of pistons and on the O2 sensors. Plugs are not the only thing building up deposits.
It keeps building up over time and is harsh on metals. The whole reason to run it is for detonation resistance. When you stop running it, these deposits that are left behind create "Hot Spots" in the combustion chambers and cause the car to detonate easier and even worse! The exact thing you were trying to avoid in the first place!
So if you start running it, you are kind of locked in to running it or some sort of octane booster after that unless you clean the build up off and start over.
I will take the risk. Besides any motor not perfectly spot on tuned and/or maintained over time (and due to poor fuel quality) will build up and leave carbon deposits in the combustion chamber and valves that can continue to burn causing the same hot spots you discuss above. That causes detonation too without ever using MMT boosters.