Full Idea
Gricean implicature theory might suggest that a disjunction is never assertable when a disjunct is (though actually the disjunction might be 'pertinent') - but the procedure is indispensable in mathematical practice.
Gist of Idea
Asserting a disjunction from one disjunct seems odd, but can be sensible, and needed in maths
Source
John P. Burgess (Philosophical Logic [2009], 5.2)
Book Reference
Burgess,John P.: 'Philosophical Logic' [Princeton 2009], p.102
A Reaction
He gives an example of a proof in maths which needs it, and an unusual conversational occasion where it makes sense.