I don't know about that JL. Sometimes it's about texture too. For instance, a killer silver-grey pair of velvet ##### can go really nicely with a light, flow-y silver gray top. Basically, I think the monochrome look can be just fine as long as the textures are contrasting and the look one is aiming for is minimal. If it's all frilly and shit, or we're talking multi-layered, then no: monochrome is not okay.
I actually took two years of costume design, so I know a lot of theory on what goes together based on colour wheels and shit... Also, in class we talked about the fact jeans are neutral in the North American wardrobe - in most cases they can be read as "black" or "blank" casual, though this used to not be the case. But shoes, bags and belts all fall into the accessory category and are a lot more of a gray area. I was watching some fashion show and the host was yelling "Your bag should not match your shoes people!" Which I found confusing.