When headsets weren't used, the cockpit's noisy environment encouraged an economy of words to make a point, which was helpful to students whose brains were working at full capacity just to keep the airplane upright. These sonic limits also promoted the use of hand signals by the instructor. A chatty instructor's behavior was at least tempered by the lack of headsets.
Now, instead of an instructor simply moving the palm of his hand upward to signal a climb, he can easily say, "Let me tell you a bunch of good things--some that you'll never need to know--about the climb." Cockpit chatter uses those scarce brain-processing cycles and ends up hurting instead of promoting student education. In these instances, less is often better when it comes to an instructor chatting away during a flight lesson.
The question is, how does an instructor know if he or she chats too much? There's really only one person who can answer that question, and that's your student. So ask him whether you chat too little or too much or just about enough. I think you'll be surprised to find that most folks want less, not more, in-flight chatter. And this isn't because they don't like what you have to say. It's only because too much chatter distracts them from practicing the lesson's objectives.