Consider the Essay as a political pamphlet on the Revolution side, and the fact that it was the Whig gospel for a century, and you will see its working merit.
Have you ever found any logical reason why mutual promises are sufficient consideration for one another (like the two lean horses of a Calcutta hack who can only just stand together)? I have not.
If you deny that any principles of conduct at all are common to and admitted by all men who try to behave reasonably - well, I don't see how you can have any ethics or any ethical background for law.
Not that pleading can be taken as a test, for the forms of action, notably Debt, ignore the fundamental difference between duties imposed by law and duties created by the will of the parties.