There's some debate on what the dew claw is 100% for, but it's often believed to give more traction when running (some dogs' dew claw would reach the ground when running) climbing, or when holding on to things.
My little girl has her dew claws and has never had any issues. The same can be said for any dogs we have had growing up. It's not common practice to remove them here (as far as I know) and tail docking is now not allowed also (Thankfully! Nothing better than a waggy tail!).
I wish I could give more information on whether a dew claw could grow back or not. Maybe a call to the vet would clear it up for you
