When looking at designs of these types, I find it's instructive to think about what catastrophic event the designers are trying to mitigate. In this case, my gu

When looking at designs of these types, I find it's instructive to think about what catastrophic event the designers are trying to mitigate. In this case, my guess is some sort of decompression where the pressure inside and outside of a window is rapidly equalized and the people onboard don't have time to react in an intelligent fashion. I think the purpose of the hole is to ensure that stress does not build up on the redundant pane so that it is only used in the case where the main pane fails. When you build redundancy into a system, it's important to ensure that failure of both pieces doesn't happen at the same time. Either can fail on its own, but they can't both fail. So you want the middle window to be as close to the ideal condition as possible when the outer window fails. By adding the hole, you accomplish this. Moreover, once the outer window fails, the hole ensures that the middle doesn't need to take the full pressure differential and allows the pressure to equalized in a somewhat controlled manner. Yes, the pressure in the cabin will drop, but there are oxygen masks that will drop if it gets too low and the pilots can always descend to a lower altitude. The people onboard will have the time necessary to take both necessary actions.