First of all, in metallurgy stronger has a specific meaning, to resist deformation. Differential hardening can indeed be done to display a hamon or quench line but it primarily, to my way if thinking, creates a tougher spine, which means in metallurgy to resist breaking. Is it useful in heat treating a thin blade? Maybe. A lot depends on the intended use of the blade. The method of differential hardening chosen is dictated by the alloy used to make the knife.
Doug