Well, I agree with you on some points : Some genre may not need a real prototype to test the gameplay. I was thinking especially about RPG's (unless you innove in the field of combat system). But I guess in every genre you can add a special thing that makes your game special. RTS for example, you may need to test a lot to find the balance between macro and combats.
Upon all, I think this is better to go with a prototype, just like a painter would go with a sketch before lining and colouring his drawing. It could give you a better overview of what time it will take for instance. Also it is sometimes easier to go step by step, little by little, creating your game, especially if you're very demanding with yourself. I know by experience I work quicker when I say to myself "Come on, just write, don't think to much, it's just a raw shot. You'll do better later". If i wanna try too perfect on the first shot it kinda paralyses me cuz i never feel like it's good. On the other hand when i go back on editing my prototype I can find the results better than I could have imagined. All I need to do then is just polish a little, and process again.