Yes you need them, why? Crankcase ventilation (PCV) contains engine oil vapor. The crankcase is vented into the intake manifold to burn along with air/fuel charge. While driving, especially spirited driving, oil is drawn into the intake and oil can start deposits on hot valves and pistons. These deposits are bad and limit engine power and efficiency.
The JLT are the easiest, but are not Cal Emission Certified. This means that places like California can make you remove them (if they even knew what they were...). Fords product is certified, but a little more trouble (3 screws...) at oil change time. Both banks vent, so both banks should get a can. A smart add.