On just about every house I've seen here in the west (CA) the vent stacks are just a bare pipe sticking out of the roof. There is no hat on them. I understand that all the vent stack does is vent the house's sewage system so that water flows downward unimpeded by suction, and since it is directly attached to the sewage system, there is no obvious reason to place a hat on it; so what if rainwater falls into it?
This having been said, when wind blows over the top of a vent stack, good old Bernoulli creates somewhat of a vacuum, depending on the velocity of the wind. The stronger the wind, the stronger the vacuum. This vacuum can draw the water out of the bowl of a toilet, which in turn would allow sewer gasses to enter the home. On some extremely windy days, I've seen the toilet bowl almost empty of water, though it has never actually been drawn down to completely empty. I imagine it would take a pretty strong wind to completely empty the toilet bowl, but I wouldn't characterize that as being completely impossible.
I hat placed on top of a vent stack that shields the top from direct wind but leaves it otherwise open to the air would prevent the Bernoulli Effect. It would also prevent birds from setting up housekeeping on top of the vent stack. So, why aren't vent stack hats more common? (I'm assuming they are used in some places.)