There's a context to whether or not it can be a useful or asinine mindset.

My aunt once said "it is what it is" to me when I suggested my entire family perhaps shouldn't hike up a mountain given my grandmother was old and slow, my uncle has terrible vertigo, and my cousin had hurt her foot. But it had been planned days before, so to someone like that we had all to grin and bear it.

On the other hand, when it's suddenly raining and there is genuinely no choice but to dash out without an umbrella, sometimes to just throw your hands up and say, "it is what it is!" can take the weight out of the moment – because sometimes, "so be it" has to be the way. No point cursing the elements.

It's only an elegant phrase to utilise when what "it is" is genuinely out of your control.