I agree with the case for using extend to apply bootstrap styles to elements that don't semantically have that meaning (e.g. this case where we are extending .label on the li I prefer to keep the class out of the DOM).

That's where I have typically drawn the line in the past (and typically not so much w/ the column classes, although that was mostly pre Bootstrap 3). The concern I have mainly is how easy the CSS is to understand (from the inspector) and how easy issues end up being to debug (based on how much extending we do).

Also, as an aside, in many cases I have taken a sweep over the CSS after things in the UI have settled down a bit and refactor things into logical mixins.  It can be difficult to look ahead too far into the patterns that the UI will need simply from the mockups (although there are some obvious areas we can address up front).