I would agree that a lot of projects need the concept of 2π, but doubt seriously that many of them would know that there exists a constant that is equal to this value (I certainly didn’t). Consider if you asked some number of people, “how do you compute the area of a circle?”. I am quite sure that 99% of the people will come up with 2π r and not τr.
It wouldn’t really hurt anything to add it, but I doubt it would be used very often.