Page 2 of 1st edition AD&D DMG: "Is Dungeon Mastering an art or a science?" This is a direct quote taken from the foreword to that book written by Mike Carr, TSR Games & Rules Editor, 16 May 1979.
Another quote I am fond of, "This game is unlike chess in that the rules are not cut and dried. In many places they are guidelines and suggested methods only." 1st edition PHB, page 8,paragraph 4, E. Gary Gygax.
Everytime I hear about how the rule book says this or the rule books are restricting my creativity as a DM I point these quotes out. They are guidelines people, not etched in stone. If something doesn't work for you and your players then don't use it! These quotes may not appear in later editions but they still hold true. It is the DM who must decide what he/she wants in their game.That is the absolute beauty of D&D to start with. Every campaign, every game played around the table with the miniatures, maps, pizza and beer, even unto the games played now online, each is as individualistic as the DM running the game. If you want fighters who can cast spells tinker with the rules till you have something thats workable. If you decide there will be no attacks of opportunity in your world then black that section out in the rules books. Whatever you do be consistent and make sure your players know what you are up to. And above all else, have fun. Thats what its all about.