You are partially wrong. It's definately useful for a while around feudalism, especially because around that time you unlock triangle farms and lumbermills. However pretty soon you'll have most tiles improved and the policies that double adjacency modifiers and policies that boost your trade routes become stronger. They give you things extra builder charges can't give you. Still, serfdom is great.
If you think this policy is really powerful by the way, play china. their builders have 4 charges instead of 3.