Kelly,
Yes, you need to know them.
Remember that the PMP exam is not 100% based on the PMBOK Guide. And remember also that the PMBOK Guide is "just a guide". It does not contain 100% of all the knowledge about project management. And it's also correct that we do not mention all the formulas in the PrepCast - that's what the formula study guide is for.
Now the trouble with the formulas is such that if you ask 100 exam takers about formulas then about a third will tell you that they think studying formulas for the exam was a waste of their time, another third will tell you that they needed some formulas and the final third feels that knowing the formulas was absolutely key for them in order to pass the exam.
In other words you never know what formulas you will encounter on the exam. But in order to be prepared we recommend studying them.