Quoted from metallik:Double flips without a wall behind the second flipper provide more challenge as you can drain between them. With games such as Paragon and Harlem, some shots are better taken from one or the other lower flippers.
Agreed, double flippers where you can drain between them are very cool. It provides a much different strategy, since you can't trap the ball. Sinbad is a great one for this, no inlanes and double flippers. You have to learn to play the bounce off the flippers and swing on the fly.