I think it's kind of neat that these docs reference back to Walski et al. (1994).
It's hard to get that kind of velocity, on the order of 1.5 m/s depending on slope. If the flow doesn't persist liong enough, you may not fill the downward sloping section.
Not only do you need air valve but you need to maintain them. They tend to get clogged up with grease and other nasty stuff and nobody wants to clean them. If neglected, they will seriously reduce the capacity of the force main.