Nobody, except the individual that has to change them all when you need to swap out frameworks or upgrade to a version that breaks compatibility or the product team decides on a new layout across the app...
Fair point, but I'm with you on it—I'm not proposing Semantic UI is any better, just that ".col-md-12" has its own set of problems. I suppose in theory what would be the best is using application selectors (i.e. class names your application owns) with the sass version of bootstrap/semantic/etc to extend the selectors with appropriate styles. However, I've done this, and while I'm no CSS expert so I probably was doing something wrong, my stylesheets became ginormous.
Now, I just kind of live with the ".col-md-12" business...
