Speed and agility are two different things. Being the most agile class, Hunters can quickly change direction, much faster than any other class, which is significant when it comes to evading and being flat out tough to pin down. Titans and Warlocks are far less efficient at changing direction and that's a bigger advantage in the majority of situations.
English
-
Sadly, agility does include speed. Yes hunters are easily able to dodge attacks, but titans can do the same with their exotics, and still be the fastest class.
-
It isn't inherent in the class though.