When I first started working with language teachers on the use of virtual classroom technology, one question that I was asked quite often is why a headset is necessary, especially if the computer has a really great internal microphone. Here are some of the answers:
When one is only talking on Skype using the computer internal microphone might work if the teacher is in a very quiet office with no interruption. Skype has a really good noise cancellation system. However, in most virtual classrooms, when one is not using a headset, there is a chance that there will be an echo unless you are muted all the time. It is very distracting to hear yourself as you are talking or a few seconds later. I have personally done several presentations with people new to virtual classroom technology and as I was speaking, I would hear myself through the person's internal speaker. If the person would have been wearing a headset, I would not have been distracted. I did get used to it, but it was distracting and difficult to concentrate.
Another reason a headset and microphone is suggested when teaching in a virtual classroom is because of privacy. If someone walks in, he/she can hear everything that is being said. I don't know about you, but if I were a student, I wouldn't want to broadcast my efforts at learning a language with everyone in the room--only with my teacher.
A final reason that a headset and microphone is suggested in a virtual classroom is that it is more professional! It gives the student a perception that you truly know what you are doing in the classroom as you have all of the tools. Additionally, as an employer, I wouldn't hire someone who does not invest in this specific tool.