The jump disrupts the motion to the net, and many players today land on one foot and then immediately hop onto two feet...hard to run to net like that.
Without a jump, the serve motion itself is a movement toward net, the jump discontinues the movement to net.
I am now looking at some examples which I will post. Not every player jumps. Michael Stich did not jump, and Ashe's jump was hardly noticeable.
I am familiar with this video, and if anything, it validates my point.
Of the Top 10 players listed, two need to be excluded (Roche, Laver), because they learned their service technique at a time (pre-1959) when the rule stipulated that one foot had to stay on the ground during the service motion. Arthur Ashe would have learned the same technique for the same reason.
Of the 8 players remaining:
- all of them, to the exception of Becker, land on their front foot (left for right handed players, right for Mac). Becker is the only one to use a scissor motion where the back leg comes in front during the jump, to land on his right foot
- all of them jump, meaning that both their feet leave the ground. It is less pronounced for Stich than for the others, but still quite noticeable for Mac
If jumping and landing on the front foot was so detrimental to the movement towards the net, all these top S&V players would not be using this technique.
But maybe the answer is that this technique allows for a more powerful serve, and they were all happy to have a slightly less balanced motion as a trade-off for it.