Filmer is famous for being "Locke's Straw Man," and with a certain amount of justification. His reasoning is a bit sloppy and his concept of the state and of the foundation of political obligation is not completely accurate. And yet, until one sees Locke's radicalization of the notion of the state, Filmer's description of late-renaissance/early modern political authority does seem to have a certain "common-sense" appeal. His reasoning probably does correspond to the political common-wisdom of the period prior to 1640, and his criticism of democracy probably does accurately reflect the view that most casual observers would have held in an age before the most powerful states in the world had adopted representative democracy.
We're used to thinking the democracy is the one true system of government -- and so, ultimately, careful study and reasoning reveals it to be. But starting from that premise exposes us to the risk of forgetting that people of honor courage and patriotism can and have served states that have not yet achieved democracy. Filmer's work -- like Aristedes's "To Rome", and perhaps even like the "Agricola" -- helps us to overcome that prejudice, to see how great leaders could have understood their work in the service of an imperfect state.