Cuz thats what you do when you have no more come backs
P.S- If they already have flawless grammar and spelling...maybe throwing in a couple of "darl"s or "hun"s to sound older and more sophisticated then the other person
EDIT: And I think people are missing the point of the statement lol. Just cuz someones gramar or spelling isn't perfect/2nd language/typo etc. does not mean that the view they have is invalid, if all you can pick on is their spelling then they have made a very good argument and obviously you have no relevant reply other than to change the subject and nit pick on minute details.
Think of it this way- IRL, If you are having a class debate...would it be ok to start correcting your opponents speech because they have a slight slur or a different accent, and completely ignore their argument? lol