I think Way of the Rat will make sense any time you want to collect many copies of a spammable card, and playing a copy of the spammable card gives you a lesser effect than buying another. As mentioned, Grand Market is the most obvious example of this.
The other significant category is cards with below-the-line effects. Using it on Highway or Haggler could easily make sense.
As a minor special case, if you throne a throne, you could throne some stuff, then use Way of the Rat on your final invocation of the throned throne to get yourself another throne.