I guess some people do use it, but I personally do not know a single person like that and as such I can only guess that it's used by only a handful of people at most. As such it'll be tough to find anyone willing to code an application for that completely for free.

The most common reason why it isn't used is because carriers charge so insanely huge premium for every second and because it's usually laggy, horrible picture quality, and can't be used between a PC and a phone. These atleast among those people I know personally, I obviously can't say much about people I don't know..