If you think about it you can see why you might not hear frank very often in spoken language. Describing a conversation or a person as frank implies that in other circumstances they are not. That is, the participants were being open and expressing their thoughts which implies that is unusual.
Diplomatic meetings, negotiations and political conferences are often described as having been frank. Which is to say they consisted of name calling, tantrums and declarations like ..this means war.. To put a positive spin on it spokesmen will often say ....the meetings were constructive because they were frank.... Which they truly were.