Explaining how a principled Reformed person ought to think about the question of formal liturgy, Carl Trueman says:
The difference is not between churches who have liturgies and churches who do not; it is between churches who have intelligent ones that are theologically informed, which they acknowledge and upon which they reflect, and those who do not. Whether one writes them down or not, and indeed how elaborate they are, is irrelevant when it comes to the question of formalism. Formalism is a matter of the heart, not of the written page. After all, unless one speaks in tongues, one is probably using written liturgical tools such as psalms, hymns and spiritual songs.
This is exactly right, and we shouldn’t miss the fact that both proponents and critics of “liturgy” often end up sharing the same flawed assumption– that liturgy is something other than a prudential application of biblical wisdom by particular people. Instead, both the advocate and opponent assume some sort of sacred law and proceed to build their case that way, supposing that the liturgy in and of itself is either required or forbidden. But on magisterial Protestant grounds, the forms are dependent upon their intent and use.