I think here the relevant definition of 'obvious' is 'readily inventable independently by competent parties.' I.e. ideally, you want the defensive patent to be

I think here the relevant definition of 'obvious' is 'readily inventable independently by competent parties.' I.e. ideally, you want the defensive patent to be obvious enough that your likely attackers will have come up with and used it in vital processes. Yes, usefulness is an important correlation, but obviousness is too, insofar as obviousness is inversely proprortional to how widespread a method is.