In my experience, even if you are building a quick'n'dirty prototype, you'll always need to fire up a terminal or run the GUI to interact/play with the prototype you are building, to see if it "works".
In this case you may apply a --this is my very own definition-- "lightweight TDD approach" just to automate all the procedures that you normally do to test your prototype. Moreover, since you are writing a test for something that does not exist, it may help you defining better what you want from your experimental code, because you are using your prototype before it even exists :)
In my experience, even if you are building a quick'n'dirty prototype, you'll always need to fire up a terminal or run the GUI to interact/play with the prototype you are building, to see if it "works".
In this case you may apply a --this is my very own definition-- "lightweight TDD approach" just to automate all the procedures that you normally do to test your prototype. Moreover, since you are writing a test for something that does not exist, it may help you defining better what you want from your experimental code, because you are using your prototype before it even exists :)