This is really an argument between two kinds of prayer–vertical and horizontal. Vertical prayer is private, directed upward toward heaven. It need not be spoken aloud, because God is a spirit and has no ears. Horizontal prayer must always be audible, because its purpose is not to be heard by God, but to be heard by fellow men standing within earshot. It serves one of two purposes: to encourage me to join them, or to make me feel excluded.
Roger Ebert tears into the ‘prayer in school’ farce: