Providing protection for the weakest members of the horde, Unrelenting turns are often the only hope for Cave Goblins to reach critical mass and you should probably play both copies every game. Even if it protects just one unit it may still be worth it.  This event can often feel oppressive to the opponent. 
It can be hard to predict if it would be even more useful next turn, so don't hesitate to be liberal in its use and play it even on seemingly little value. Cave Goblins hand space is very limited.
Just don’t leave Sneeks too exposed on an Unrelenting turn as playing this can tempt your opponent to ignore your commons and use all of their firepower on Sneeks.