Those type of services are the ones I truly love when everyone in the church is on one accord. Why is it that when there is that type of service and the pastor/preacher doesn't get to preach there is always complaints about the preacher preaching. If God gave a message to the preacher, he might not deliver it through a (scripture text, topic) pattern, but however the Lord leads.
Conclusively, I love those type of services because the Spirit is moving strong and in a mighty way.