I think an honest person doesn't need God to back up his words. Your words alone can justify what you are saying. It also makes the peson less credible for me when he had to use someone like God to make a point. Most of the time I think he is lying when he says such. It makes me even furious when he uses that phrase even when he deliberately making a lie.
Like the saying in Tagalog, "Ang salita ay sumpa. (Words are promises.)" we should say true to our words so that whatever you say makes you unto what you are and thus more credible.