I'm all for winging it when you don't know a rule, just to keep the game going, and for improvisation in general. But I for the life of me can't understand the outright hostility that many folks have for the GM knowing and following the rules, and having a book directed at the GM for applying and adjudicating said rules. If the people that complained about how useless the DMG was actually bothered to read it, they would realize how foolish they sounded. No, it is not comprehensive and not aimed at expert GMs (we need a DMG2 for that) but it isn't a bad or unnecessary book.
Also -- if you are running 5E "exactly like" you ran 1E and 3E, you are doing your game and your players a disservice since all of those are in fact different games.
Also -- if you are running 5E "exactly like" you ran 1E and 3E, you are doing your game and your players a disservice since all of those are in fact different games.