1) If you do not allow volunteers to work in the ministry you will kill the Christian principle of sacrifice in the church.

Matthew 16:24 

Ephesians 4:11-12

2) If you do not allow laymen to work in the ministry you will remove the opportunity for people to demonstrate faithfulness.

Luke 16:10-12

3) If you do not allow lay people to work in the ministry you will employ people to do jobs that do not occupy them fully. 

4) If you do not allow lay people to work in the ministry everything done in the church will be related to money.

Ecclesiastes 5:10

5) If you do not allow laymen to work in the ministry people will not learn obedience and submission. 

John 21:18-19