The zippers are fairly waterproof, but the fabric is water-resistant at best. It will bead & shed water to a point, but eventually get saturated. So really the biggest threat is rain soaking through the fabric, not sneaking through zippers. I'm not really a big fan of zipping it all up, either, just because that encumbers access. Just leaving the putter pocket out but flapping open, and same for the main disc compartment cover, provides a good combination of weather protection vs. access.
Another thing to consider is the umbrella holding side pocket on the Octo. I'm not a big fan of umbrellas, because rain is so often accompanied by wind. However, in calm rainy conditions, the umbrella holder will protect the bag from rainfall. (Although it doesn't cover the bottom of the pack when you set it down in a wet area, which is another benefit of the rain fly.)
Another thing to consider is the umbrella holding side pocket on the Octo. I'm not a big fan of umbrellas, because rain is so often accompanied by wind. However, in calm rainy conditions, the umbrella holder will protect the bag from rainfall. (Although it doesn't cover the bottom of the pack when you set it down in a wet area, which is another benefit of the rain fly.)