"But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you. And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words. Do not be like them, for your Father knows what you need before you ask him." - Matthew 6:6-8
We are instructed to pray continuously (1 Thessalonians, 5:17), but that order seems meaningless if it means we are to stand silently as someone else prays in our stead, or carry around a lengthy tome of prayers to be read in unison. The whole point of prayer is to communicate with God. In prayer we speak to our parents about how things are going in school, or what difficult decision we are about to make in life. Choral performances are for audiences, not for personal conversations. Audiences do not respond to the content of the words, nor can they provide meaningful feedback. The words are not personal in a choral endeavor, or in one person praying on behalf of a group. A speaker is designated in a political forum, not in a family. By praying in this way, we sacrifice personal communication and our relationship with God in the hopes that generic things said "correctly" will have more impact. It feeds our fear by allowing us to avoid the divine. It transforms God into a politician watching a play rather than a parent watching a child play.
Prayer should be, as Jesus instructs us, a private and personal interaction. An honest moment for us to communicate with God to request something of importance, seek wisdom, or simply continue our relationship. Praying together can do none of these things. There is power in prayer, and there is great power in a great many people praying for the same thing, but they must each pray it for themselves.