We had to go through progesterone shots and a bunch of other stuff with both of our pregnancies too.
While the progesterone shots will help, the cerclage is an extra safety measure. Yes, God will help along the way too. Consider this: Without the cerclage, your baby may decide to come out sooner than she's ready...would you rather she stay "in the oven" a little longer and be able to go home with you when you're ready to go home or have to stay in intensive care and have it be a while before she comes home? The longer she stays in, the better it is for her.
I would do it..."better safe than sorry".