I don't believe that the tread depth actually is the deciding factor. By the time you wear the tires down that far they've had a LOT of heat cycles put on them. As a tire gets heat cycled repeatedly, it gets harder and looses grip. That's what I understand to be the real issue in those tests.
As far as the depth to replace at, that's been debated quite a bit. Tire shops in many areas are saying 4/32" for tires that never see snow and 6/32" for tires that do get used in the snow. Some states in the USA have bumped standards up to 3/32" as well.
On the other side, most states are still at the 2/32" wearbars as the standard. Environmental and conservation concerns are pushing others to go with the 2/32" and "don't be stupid" when driving in the rain.
The main risk of insufficient tread depth in rain is hydroplaning at speed. If you're not going fast enough to hydroplane, there's no real benefit to throwing out tires early. If you only drive slowly in occasional light snow, same idea.
Best option would be to replace tires daily, but that's just silly. :-) So, you have to pick the appropriate cost/risk value that fits within legal limits.