As I’ve opined at various places, I think a congregation will REALLY only sing what they know by heart. That is, overwhelmingly, the most important issue (just my opinion, of course). Congregations differ on how much they can know by heart and there are differing ways to impart that knowledge. However, often there is little or no effort given by music leaders towards helping the congregation learn songs by heart. One could argue that overhead projection and congregational hymnals do not really help a congregation with this; further, unless the repertoire is carefully controlled, the congregation can not keep up with the music leaders who practice extensively.
Similarly, what one sings to babies is more important than what they hear from an iPod player.