You don't have to run a Watts linkage or Panhard bar with a triangulated 4 link..... But it does keep the car "hooked up" better in turns. For the street, a Watt's linkage can use urethene bushed ends which kills the noise of heim joints. In normal driving, probably never even notice the Watt's linkage is there but when you start really stuffing the car into turns, you'll definitely notice the increased stability. Leaf springs and curvy roads shouldn't even be in the same sentence....no tunability in ride or handling with a leaf spring. With coilovers you can change the spring rate, go to a progressive rate coil, change the ride height, play with the shock valving, etc. etc. Do you need all this on the street for normal driving???? Heck no. But if we only put the things on a car it had to have, it wouldn't even be a Hot Rod, would it????