The key issue on whether you can reuse a pin is whether you have hyperglide, or a similar cassette under another name). The cut-away teeth, and shift ramps make shifting much smoother, and without them modern index systems wouldn't work well. These systems also allow shifting under load which is where the problem arises. Aggressive shifting, especially under load, puts tremendous side load on the plates and causes broken chains by pushing the plates out on the pins until they fall off. This is worsened by narrower chains whose pins are almost flush leaving zero room for outward plate movement.

Some 20 years ago, chain makers started peening the ends of the pins over the plates like a rivet to keep the plates from moving out. That pretty much solved the problem except for the most extreme cases, but also meant that you couldn't push pins back and forth. So any 8s or more chain needs to be closed with a special pin, or a connecting link to maintain integrity at the splice.

Will it break immediately if you push the pin back? No, it's as strong as it ever was. That is until you shift under a bit of load as the splice comes onto the cassette (roughly 3 or 4 out of 114 times). Then the plate will be pushed out a bit. Do this a few times and the plate will be hanging by a hair, then when you climb your next hill, it'll let go.

So it you're a light rider, who shifts smoothly, and never under load, odds favor you. But, as anyone who's ever broken one can tell you, a chain will always break at the worst possible moment, when you're putting maximum load on the pedals and it's no fun.