Air venting into the milk stream is very important
Try filling a tank using a fuel can with out having the air vent open on the container! The fuel will not flow very well and will take much longer than if the vent to allow air into the container has been open. The air vent allows the fuel to flow in a faster steady stream.
A similar situation occurs during milking, and there are several ways to introduce air into the milk line. A vented claw is a way that has been used for years. Today, with more knowledge of flow properties, a number of inflation manufacturers have placed air vents in the milk tube just below the bottom of the shell. Others have placed the vent into a seperate nipple just below the bottom of the shell. Another places an air vent into what is called the "vacuum reserve area" of the mouthpiece, starting with the mouth and going down the throat of the inflation.
Logically placing the vent into the "vacuum reserve area" of the inflation would seem to defeat its purpose. It is called the "vacuum reserve area" because it helps keep suction vacuum at a sufficient level to hold the inflation on the teat during the massage phase. Logic set aside, this approach works on a large number of farms.
There are several designs of air vents, the best design for preventing clogging is that of the Hybrid® plastic nipple that has a very thin wall in which the vent penetrates through. This design has almost no clogging. The second best is that of Bou-Matic which even though it has some length to the vent it has a small entrance hole to a larger exit hole inside the tapered vent hole. This is their patented feature.