Earlier quoted context omitted.
Couldn't disagree more with your statement. What if you have a sidebar that appears 20+ times throughout your application in different contexts. You might decide it is too wide. Are you going to go through all your templates and change class="col-md-blah" or are you going to change one mixin? Also, if you are providing a theme, I don't see how you lose portability. Your mixins will be relying on base bootstrap anyway…
For pretty much anything anyone could describe as a "web application" if you have the HTML for your sidebar copied around 20+ pages instead of stored in a common template somewhere, you have bigger problems than whether your HTML is semantic.
The example was contrived to prove the point. I agree that the specifics of the Scenario would be indicative of poor practice in other areas, but that doesn't change the fact that the markup should be semantic.