Therapeutic modalities had an entire course devoted to it in my educational curriculum and there is more and more research being done on the topic. There is plenty to learn about the theory and application of each individual type of modality from ultrasound to electrical stimulation and plain old ice.
I definitely learned a lot and the use of modalities has been definitely beneficial for me as a clinician, especially for acute injuries. However, I have found myself and others at fault for relying upon modalities too much. There are too many clinicians relying upon the "powers" of modalities instead of using the power of movement and rehabilitative exercises. I'm not saying that they don't have their place because they definitely do but they often are too heavily relied upon.