I've got an Engel and love the fact I can't spill anymore. As far as a vent here's another perspective. I'm a prolific fizzy water drinker so the kids bought me a soda stream for my birthday. I've noticed two things: First, it works better if the gas jet is just out of the water as opposed to submerged. Second the carbonating vessel is "sort of" sealed for the fizzing process.
I'll guess if the gas, in our case air, is passed through the liquid and the remaining space is somewhat under pressure more of the gas tends to dissolve into the water rather than escape into the "empty space" so a vent would be counter optimum aeration.
Just a guess... Plus if you're sealing to prevent spillage why leave a hole?