Full Idea
A circular argument - one whose conclusion is to be found among its premises - is inadmissible in most informal contexts, even though it is formally valid.
Gist of Idea
Circular arguments are formally valid, though informally inadmissible
Source
Robert Hanna (Rationality and Logic [2006], 2.1)
Book Reference
Hanna,Robert: 'Rationality and Logic' [MIT 2006], p.31
A Reaction
Presumably this is a matter of conversational implicature - that you are under a conventional obligation to say things which go somewhere, rather than circling around their starting place.