It's all about personal taste. An ///M badge at the back adds little visual value to your vehicle, and it just makes you seem like you're trying to trick people into thinking you have an ///M car. This is how people will always see it, EVEN if you aren't trying to, and are just proud if your car and like the badge.
However, bumpers like an M3 bumper replica on a normal 3 series, or a 1M bumper on a 1 series (which a lot of people put on around here and don't get vlak for) is not the same as an ///M badge because you are visually changing a major aspect of your car because you prefer the look. It's a visual upgrade because you think it looks better. Same story as the kidney grill stripes. It looks better with it to some people, its not about pretending to be something different.