Just yesterday Lsmkenpo made this interesting observation in another currently running thread
The legs are the key to kenetic chain on serve: Agree or disagree?.
"I would estimate about 90% of the intermediate to advanced recreational players don't achieve a proper coiled position on their serve, therefore leg drive isn't adding much to their serves to begin with. Leg drive without a coil is just jumping up and hitting the ball at a higher angle, and arming the serve. As the player tires during a match their jump height decreases and so goes the timing and consistency of their serve with it.
The legs do facilitate the kinetic chain, but how they should start the chain is often misunderstood. They don't facilitate solely by bending to drive up and hit the ball, they start the chain by bending to allow a stronger coiled position, big difference and probably one of the most misunderstood aspects of the proper service motion.
Think of the knee bend as the distance between the coils of a spring that is anchored and standing upright. If we pull the top of the spring back to store some energy the distance between the coils on the backside of the spring are shortened, here is the equivalent to the service knee bend. We are not storing energy by pushing the spring straight down from the top and allowing it to spring up, We want to pull the spring back deeper (knee bend)to store more energy to allow a stronger arc into the ball, this is how the knee bend adds the most power to the serve.
Here is a great video that really shows how the knee bend is used to coil, the deeper Soderling bends his knees the more he coils, thus storing more energy to propel into the ball.
http://www.youtube.com/watch?v=a56pvP1i6x8
So the answer to the question depends on how the legs are being used, if they are being used to facilitate a stronger coil than they are adding to the serve, if they are being used to jump up without adding to the coil, it is not nearly as beneficial to the serve.
I know a lot of players might think they are coiling, but most would probably be surprised how limited their coil and arc into the ball actually is compared to a pro level serve. When you think you are coiled enough you probably still have a good ways to go. This is why pros can make a 100 mph+ serve look effortless and amateurs look like they are tapped out around that speed."
-http://tt.tennis-warehouse.com/showthread.php?t=359477&page=2
And the concept of coil down [knee bend] in the backswing, coil up in the forward swing is every bit as important in the forehand, backhand and volley as in the serve.
Stand straight up with the knees locked and you will look awkward trying to wave at the ball with your arm.
Coil down and coil up and your strokes will flow. Your whole body will flow.