I would say that replace is a bit of a misnomer. The text for bestial rage says "when you rage..." so you still have the rage action. The bestial rage feature just adds more to it. That said the "when you rage..." text does seem to indicate that you must always add on bestial rage's additional features each time you rage.
Bestial Rage is the initial Instinct ability for Animal Instinct Barbarian. It is more that it is an add-on definition of how Rage works for that subtype of Barbarian.
That said, I can't think of many reasons that you wouldn't want to use it. Maybe if you were already under a Morph effect that would conflict and didn't want to lose.
So what scenario do you have in mind where you are hoping that Bestial Rage is optional?