There is also a general idea behind creating a story which needs to be enough to make yourself a salary if you know what I mean. Kishi needs to create these illusions for the readers, something that leaves a cliffhanger behind for the reader to wonder about. Having the almighty Yata Mirror which can protect from anything and the Sword of Totsuka which slashes anything will impress the readers. Kishi needs to impress the readers in order for them to buy this comics. Without these 'flaws' as sort to speak, this comics wouldn't have made it in the top 3 selling comics. If Kishi would of stated and eliminate these 'flaws' Kishi wouldn't give the opportunity for the reader to wonder about "Up to what extent can that shield defend up to?" If Kishi states that the Yata Mirror is able to defend from anything, it leaves him to be bias and raises arguments which is clearly his goal. He wants us to argue about Naruto and throw our ideas around. These are all the elements he clearly places in the right order so threads like these will happen.
If Kishi would of stated an accurate measurement as up to what is the Yata Mirror can defend up to, there wouldn't be any arguments and this thread wouldn't exist neither the other thousand threads about the shield.