So, to summarize:
1.) There are too many extraneous variables, and not enough empirical data to say whether churchwardens cool the smoke/act of smoking or not. If they do, that's a plus, if not, you're right back where you would be with any other shape.
2.) Churchwardens offer increased functionality in some situations (Sitting, reading, driving, monitoring the church grounds, smoking out the rectory window). They offer decreased functionality in others (doing pretty much anything else).
3.) The final and probably most influential factor. They look nice/ugly depending on who you are. The appeal of a churchwarden lies in that it is a unique shape, just like any other. Some find it aesthetically pleasing, others do not.
Conclusion: Based on the evidence, we have come to the statistically significant scientific conclusion that a churchwarden is a tobacco pipe.