If I had to guess, I'd say whatever the amount is isn't worth holding on to for the sole purpose of having it. For example, let's say Warriors got 12.5% extra critical hit chance at 50% Beast Gauge, and 25% whenever it was full. Now THAT would be something noticeable. It's probably an extra 10% crit chance when it's full though, which is pretty good but nothing to stop you from not using that Fell Cleave.