Having just gotten back from doing a book signing tour, I've missed much on the board. Love the dialogue I've read here.
If I can add my own clarifications on this topic:
The legs are important when used correctly in getting the maximum speed and overall effect of a serve...when all the other elements of the swing path, grip and position are optimized.
Personally, I've seen some pros over emphasize the legs to beginners who had such a terrible motion that no amount of legs (done right or wrong!) would be of help.
I've mentioned my demonstrations of kneeling on my knees and serving at 100 mph...only to show that legs don't have to be emphasized to hit this speed. Yes, I serve about 120 thus one could conclude that the legs (and added leverage due to added height) would add about 20% or so. (I personally think that is generous.)
I teach many drills to incorporate the legs correctly in my students who have mastered the swing path, grip and general stance mechanics of the serve to gain more effect and pace. But, I would never teach the legs (other than the stance and general leg work within a top serve) at the expense of the swing path components.
Thus, I think many here are on the right track...but the biggest statement is probably WHEN would we teach the legs. In my experiences, I've seen where the few students who were taught with greater emphasis on the legs by some pros were far slower in developing a serve that matched their potential. Where I have had 14 year old girls serving over 100 mph, I've seen adult men who have terrible service motions fail to come close to 70 mph with any serve.
All in all, the leg drive on the serve is criticle for a player to reach their full potential in creating the fastest and most effective serve possible.
Thanks for all who also mentioned my books. I hope they are very helpful to those who read them.