Make no mistake about it: the most important musical group in the church is the congregation. Worship is for all of God's people; the New Testament doctrine of the priesthood of all believers calls for the saints to lift up their voices (whether musically gifted or not) in a unified chorus of praise and adoration to God.
However, that is not to say that gifted and trained musicians do not have an important contribution to make to the worship life of God's people. Old Testament worship gave a crucial role to the Levitical musicians, who were set apart vocationally to provide this ministry to Israel. Moody has his Sankey, Graham had his Shea: the point being that the proclamation of God's truth takes place in musical form as well as in the spoken word, and the Word of God must be central in every portion of our services.
To be sure, the standard choral contribution of call to worship/anthem/response is a bit tired, and certainly unworthy of the potential of those most gifted and equipped to prompt and model worship. I dare say, in most churches where choirs are not motivated or where morale or attendance is a problem, the cause is not that this ministry places too many demands on them, but rather that they are not being truly challenged to minister with excellence and to step up to a new level of leadership in the worship of God's people. The choir certainly fits the role of "prompter" designated by Kierkegaard for those up front (with God as the "audience" and the congregation as the "performers); choir members can use their special gifts in a powerful way in inviting and leading God's people in worship.
Support for Congregational Singing
A corollary to the fact that the congregation is the primary musical group in the church, is that the primary function of the choir is to help the congregation sing better. The choir is not to be a performing group where the musical elite of the church prepare and present aesthetic delights to the wonder of the rank and file. Rather they are to use their gifts to under-gird, support, enhance, strengthen, and embellish the corporate ministry of song.
The most obvious contribution which a choir can make is that of sonic support for the people's song: in terms of volume (especially in a room which is on the dead side acoustically), depth (strengthening the bass), fullness (adding harmony by singing in parts), and tempo (helping to set and maintain it). In addition, the presence of a choir singing in parts makes possible effective a cappella singing at climactic or reflective moments.