Re: 2nd Serve Opinion
If you don't mix up your serves, your opponent gets used to the amount of spin or pave you're throwing at him, and the effectiveness of your serve decreases. Your coach has made a good point, because throwing in a slice serve every once in a while will throw them off and keep them guessing, and it will give you free points because they won't recognize it and it will throw them off. Same reason you mix up placement, or throw a heavy kicker in as a first serve every once in a while. Its a mix up play.