I'm pretty sure this has been an issue since the Old Testament times when you had to bring a perfect sheep to the altar. "Your sheep sure is pretty perfect but not as perfect as MY sheep!"

It is not just a problem for the worship leader. Every singer, even those in the pews, might be singing so that others will hear and admire them.

My take on it is: eternal vigilance is all we can do. Every person has to remember that all they have is a gift and that all they offer is a possession of God in the first place.

Corollary problem: how do we quit judging speakers and singers, assuming they are egotists and insincere, all the while failing to judge ourselves?

Within the Lordship of Christ (who gave himself for us) we are being transformed into His image. Leaders (whether Praise Team or Bishop (I Pet 3) are first servants....to their Lord, and to those they would support and encourage through the Spirit. Given the effectiveness of that context, the discenting voices would be made indiscernable in the indescribable joy brought about through the unity we find in Him. There is no room for self-serving performance or "lording it over the flock" in a church where Jesus is our Savior, or Lord and our focus.

I have not read the book mentioned in the question above, and I guess I am slightly perplexed concerning the matter of "skill" concerning "worship." I am not trying to be critical or argumentative but I guess I never though of the idea of "skill" in the context of worship. I have a man in my congregation who has an affliction of "Down Syndrome" and he is highly functional, able to own his own home and hold down a job and drive a car. He loves to sing at church and does so quite loudly. If one were to measure his "skill" by common standards he is absolutely terrible, way off key, wrong words, and in fact he sounds like a howling animal at times, BUT I speculate that Angels in heaven stop what they are doing and in amazement listen to the 'fantastic skill" he has to worship with all his heart soul and mind. Men would snicker and laugh ( and I must admit that I too have broken a smile) when he sings, and some would like him not to sing at all, but in my mind it is a way too beautiful of worship to stop. He has "NO" skill in worship as men would measure, but I wonder how the Lord measures him?

Its like leading as a bishop -- 1 Peter 3. God's definition of leadership is different. He requires one who is so submissive to their Lord and so intent in service to His church, that the voice of critics is drowned in the joy of service in the Spirit.