If I were a student at Verbling, maybe I would exactly what many students do about Trial Classes: take them whenever I pleased, with the sole purpose of practicing a language for half hour for the lowest price available here, USD6. And, why not? There is no policy against it.
Considering that the USD6 half hour conversation session is so popular among students, why not make it a regular service and let teachers offer it as such, giving them the choice to do so?
What do you think, as a student? What do you think, as a teacher?
As a teacher, I would prefer to know if I am teaching a lesson, doing an assesment, or just having relaxed conversation with corrections (or without, whichever the student chose), and having a choice about offering that service or not. Personally, I would offer it (only short notice, so as to use my time productively when I am not booked or I have a cancellation).