The narrower diameter of the draw tube increases the velocity of the vapor stream and "organizes" it so that it moves farther as a single straight column once it exits (within the confines of the mouth). This basically means that warm vapor is more likely to arrive at the back of the mouth, causing coughing -- not good! Using the Box native provides for a larger draw hole (lower stream velocity) and because its "effective length" is very short, the vapor stream is less organized -- both effects place the vapor closer to the front of the mouth, which for various neurological reasons, mean less coughing (or none at all). It is a specific and very carefully thought out part of the Box design to ensure that the vapor channel is as short and as likely to place the vapor closer to the front of the mouth as possible -- where most of the users sensory capacities are.
Overall, the goal of the channel is to provide the maximum and most easy/natural user feedback and the maximum vapor (ie, little to no vapor condensation in the channel). One of the disadvantages of using any sort of draw stem at all is that it removes the user from directly sensing the vapor temperature and it provides a place for the vapor to condense. Condensation is bad -- it means both lost vapor (less efficient) AND it makes eventual cleaning a requirement.