I agree with Turner. Monarchy is good sometimes. An early republic can really get into trouble. But when you're done expanding and your cities begin to grow and build marketplaces, then Republic is almost always better. Even though Monarchy has no war-weariness, fighting wars in a Republic will make you a better warmonger. You have to learn to fight quick, efficient wars that accomplish a set goal.
Monarchy is still not useless. Good reasons to use Monarchy are if you have no luxuries, or you know you are going to fight a long, slow war. If I am playing a religious civ, I will use Monarchy early then Republic.
I think the biggest complaint against Monarchy would not be that it sucks, but that anarchy sucks. It's usually not worth it to go Monarchy, then Republic, then another government later with all those turns in anarchy. Playing a religious civ makes this better, but we don't always play religious civs. That's why most players get into Republic right away and stay there the rest of the game. In my games, 90% of the time I use Republic, then either stay in Republic the rest of the game, or switch to Communism when it's available.