I can provide a teacher's point of view:
I have had an Instant Tutoring session with a student before. Though they are not as frequent as regular sessions or as long (only 30 min each), they are still very convenient, particularly for the student. It can be a bit hectic for the teacher in preparing materials almost instantly for a student who wishes to request a session. It is still very convenient, however, as you can become available for Instant Tutoring whenever you have some free time. It is definitely a great new way to meet new students.