It'll look huge and ridiculous on your arm with a bumper on it, and you will look ridiculous wearing a watch with a bumper on it. Seriously, who puts a bumper on their watch? Nobody, that's who. (Who isn't a complete dork out of touch with reality, that is.)
People put bumpers (and other protection) on their phones because the risk they may accidentally drop it, but with a watch strapped to your arm there's no risk of that - unless your arm were to say, fall off, or something. In which case you probably got bigger concerns than a possible scratch on your watch...