If you stack two 3 volt batteries, the risk is that one will eventually be at a different charge level that the other. This can cause one to reverse discharge into the other. What happens when they go into runaway is that the battery the gets too hot or develops an internal short will start spewing under internal pressures. Mostly, this happens when a cell is overheated, damaged, or the liquid inside shorts due to metal particles suspended in the liquid that cause shorts within the battery.
The vent holes are there to allow that pressure to escape from the PV, but only to a point. The PV can become a flare of hot gases and when the pressure starts to rise above the capability of the vent holes to allow the pressure to escape, you get a catastrophic explosion where the case splits and explodes. It doesn't happen that often, but it makes a believer out of those who have experienced it.
Lots of people stack 3 volt Li-ion cells. Before doing this, I'd advise you to read this article. It doesn't happen that often, but the explosion or venting releases caustic liquids that cause respiratory irritation, burning of skin, and other nasty effects. There's a picture of an exploded cell phone midway the article. One article I've read says that Li-ion cells can produce 1/4 to 3/4 the explosive force of a hand grenade.
Battery Safeguards; Protection Circuits