Actually, let me tell you why. Most, but not all, players tend to play good characters. Since the MM is meant to present monsters to the players to defeat (as opposed to allies) a good aligned monster is essentially a waste of paper for them.
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The MM is a book of combat statistics. If you're not planning on fighting on killing something . . . why do you want its combat statistics?
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I suppose my point is that the MM is a book full of things to fight and kill, and that's what it should be. It's called the Monster's Manual after all, not the Allied Creature's Manual. I'm not saying that information on riding Metallic Dragons into combat against Chromatic Dragons would not be nice, but maybe it would be more at home in the PHB3 (along with information on riding Chromatic Dragons, since there are going to be Gith . . .).