Hey Foosh,
You're right, helmet radios are a great teaching tool. We have been using them for the last couple of years and they have completely changed the way we teach and the results we get.
They are essential as soon as you get up to body dragging level and beyond. As you drag away downwind, an instructor can't keep up with you which means you no longer have a clear communication with your instructor.
We can correct bad form as the lesson is progressing in "real time" whilst the student is making mistakes rather than waiting for the student to catch up with the instructor and explain the mistakes he made and then go and have another go. It cuts lesson 2 and 3 in half in respect of time taken to achieve the same results with the non radio method.
If we have radio trouble or a battery goes flat and we have to go back to the old method, even for 5 minutes whilst we change batteries, it creates stress for our instructors as we can't offer that instant calming instruction to our students to "talk them through" difficult situations, like a tangled bridle and a looping kite for instance.
For us, it's the only way to go.
Good winds,