You only need a temp controller if your using a freezer as a kegorator. If your using a refrigerator then the fridge temps will be fine for serving beer. If you have a freezer you need the controller to keep the kegs from freezing.
But you can also use a temp controller to make a fermentation fridge.
If you do decide to get a controller, I highly recommend the digital ones from ranco. I've had the analog models and it was not so great.