Onslaught charge and needing rage

[Synestra]

Onslaught has a beast gauge cost intentionally because they do not want you using it to pull.

Well i always tought that is one of the main uses for skills like charge, like throwing a tomahawk and instanty charge after that to gain burst aggro and resources. Plunge works like this so i dont really understand why they made onslaught diffirent.