Do you think O rings are redundant for quiet switches? Why?
O-rings serve part of the same purpose as the rubber bumpers inside the
silenced switches: by cushioning the key when it hits the bottom. The rubber inside a switch will cushion the slider itself while an O-ring will sit under the
keycap so that it would hit the key switch's housing first and thus prevent the slider from hitting the bottom.
Switches and keycaps were never designed for O-rings in the first place. They reduce key travel, whereas "silent" switches often are designed so as to not reduce key travel that much.
Different keycaps are made differently, and you would use different O-rings for different keycaps so that key travel does not get too reduced with them. Then there are some types of keycaps that don't fit O-rings at all (unless you stack many of them inside each keycap ...

)
Both approaches also change the key feel a bit by making it softer when you hit the bottom: but if you would use
both O-rings and a silent switch, it would become doubly softer and with even less key travel.
BTW. The rubber bumpers inside silent switches also dampen the noise of the key rebounding, which O-rings don't.